Whales evolved from small aquatic hoofed ancestors

Travel back in time to about 50 million years ago and you might catch a glimpse of a small, unassuming animal walking on slender legs tipped with hooves, by the rivers of southern Asia. It feeds on land but when it picks up signs of danger, it readily takes to the water and wades to safety.

The animal is called Indohyus (literally “India’s pig”) and though it may not look like it, it is the earliest known relative of today’s whales and dolphins. Known mostly through a few fossil teeth, a more complete skeleton was described for the first time last week by Hans Thewissen and colleagues from the Northeastern Ohio Universities. It shows what the missing link between whales and their deer-like ancestors might have looked like and how it probably behaved.Whales look so unlike other mammals that it’s hard to imagine the type of creature that they evolved from. Once they took to the water, their evolutionary journey is fairly clear. A series of incredible fossils have documented their transformation into the masterful swimmers of today’s oceans from early four-legged forms like Pakicetus and Ambulocetus (also discovered by Thewissen). But what did their ancestors look like when they still lived on land?

Bone-crushing super-wolf went extinct during last Ice Age

Being confronted with a pack of wolves is bad enough, but if you happened to be in Alaska some 12,000 years ago, things would be much, much worse. Back then, the icy forests were patrolled by a sort of super-wolf. Larger and stronger than the modern gray wolf, this beast had bigger teeth and more powerful jaws, built to kill very large prey.

This uber-wolf was discovered by Jennifer Leonard and colleagues from the University of California, Los Angeles. The group were studying the remains of ancient gray wolves, frozen in permafrost in eastern Beringia, a region that includes Alaska and northwest Canada.

These freezer-like conditions preserved the bodies very well, and the team found themselves in a unique position. They could not only analyse the bones of an extinct species, but they could extract DNA from said bones, and study its genes too.

For their first surprise, they found that these ancient wolves were genetically distinct from modern ones. They analysed mitochondrial DNA from 20 ancient wolves and none of them was a match for over 400 modern individuals. Today’s wolves are clearly not descendants of these prehistoric ones, which must have died out completely. The two groups shared a common ancestor, but lie on two separate and diverging branches on the evolutionary tree.

Hunter/scavenger

The genes were not the only differences that Leonard found. When she analysed the skulls of the Beringian wolves, she found that their heads were shorter and broader. Their jaws were deeper than usual and were filled with very large carnassials, the large meat-shearing teeth that characterise dogs, cats and other carnivores (the group, not meat-eaters in general).

The overall picture is that of a skull specially adapted to bite with tremendous force. These ancient wolves were hypercarnivores, specialised for eating only meat and killing prey much larger than themselves. Leonard even suggests that the mighty mammoths may have been on their menu.

Once prey was dismembered, the wolves would have left no bones to waste. With its large jaws, it could crush the bones of recent kills, or scavenge in times between hunts. Today, spotted hyenas lead a similar lifestyle.

The wolves’ teeth also suggest that bone-crushing was par for the course. The teeth of almost all the specimens showed significant wear and tear, and fractures were very common.

Their powerful jaws allowed the Beringian wolves to quickly gobble down carcasses, bones and all, before having to fend off the competition. And back then, the competition included many other fearsome and powerful hunters, including the American lion and the short-faced bear, the largest bear to have ever lived.

Evolution of a super-wolf

Leonard suggests that the ancestor of today’s gray wolf reached the New World by crossing the Bering land bridge from Asia to Alaska. There, it found a role as a middle-sized hunter, sandwiched between a smaller species, the coyote, and a larger one, the dire wolf.

When the large dire wolves died out, the gray wolf split into two groups. One filled the evolutionary gap left behind by the large predators by evolved stronger skulls and teeth. The other carried on in the ‘slender and fast’ mold.

But in evolution, the price of specialisation is vulnerability to extinction. When its large prey animals vanished in the Ice Age, so too did the large bone-crushing gray wolf. Its smaller and more generalised cousin, with its more varied diet, lived to hunt another day.

Similar things happened in other groups of meat-eaters. The American lion and sabre-toothed cats went extinct, but the more adaptable puma and bobcat lived on. The massive short-faced bear disappeared, while the smaller and more opportunistic brown and black bears survived.

Leonard’s findings suggests that the casualties of the last Ice Age extinction were more numerous than previously thought. What other predators still remain to be found in the permafrost?

Reference: Leonard, Vila, Fox-Dobbs, Koch. Wayne & van Valkenburgh. 2007. Megafaunal extinctions and the disappearance of a specialized wolf ecomorph. Curr Biol doi:10.1016/j.cub.2007.05.072

Technorati Tags: wolf, gray wolf, carnivore, ice age, predators,

Related posts on studying extinct animals:

Inner ear size can predict a mammal’s agility
Tracks provide evidence of swimming dinosaurs
Death of dinosaurs did not lead to rise of modern mammals
Microraptor – the dinosaur that flew like a biplane

Inner ear size can predict a mammal’s agility

The semicircular canals of an animal’s inner ear controls its sense of balance. Their size can tell us whether an animal is slow and ponderous or fast and agile. They can even help us to reconstruct the behaviour of extinct species.

Studying the way an animal moves by looking at its ears might seem like a poorly thought-out strategy. After all, short of watching it directly, most biologists would choose to look at more obvious traits like tracks, or limb bones.

But while an animal’s limbs may drive it forward, its inner ear makes sure that it doesn’t immediately fall over. By controlling balance, it plays a key role in movement, and its relative size can tell us about how agile an animal is.

Organs of balance

When we walk, the image that forms on our retinas changes quite considerably. But no matter how fast or erratically we move, our view of the world neither jerks nor judders. It’s all stable images and smooth transitions, and the inner ear plays a large role in that.

In the inner ear, three semicircular canals control our balance by acting like small gyroscopes. The canals are bony, fluid-filled tubes arranged at right angles to each other and send information to the brain about the body’s orientation.

When the body moves, so does the fluid and this sloshing is sensed by hairs in the canals and relayed to the brain. The muscles of the neck and eye tense reflexively in response to these signals, and these help to stabilise our view of the world.

In humans, the inner ear doesn’t really have to work too hard – we’re limited to moving on the ground, and not very quickly at that. It’s a whole different story for a fast and agile animal like a bat, twisting and turning in three-dimensional airspace while avoiding obstacles and predators.

Acrobatics vs. stealth

Fred Spoor from University College London and colleagues from around the world reasoned that these different movement styles must be reflected in the size of a species’ balance organs. There is some evidence for this already – the practically immobile sloths have small semicircular canals, while manoeuvrable birds have relatively large ones.

But these findings seem almost anecdotal compared to the massive amount of data that Spoor collected. His group looked at the canals of 91 different species of primates, representing all the major families.

The primates are an ideal group for this type of analysis – despite being closely related, they have a vast range of different movement styles.

Acrobatic gibbons swing through jungle canopies at high speed using ball-and-socket-jointed wrists (top). At the other end of the spectrum, the appropriately named slow loris is a ponderous and stealthy climber (bottom).

The group used a special CT scanner, a hundred times more sensitive than those used by hospitals, to build detailed 3-D reconstructions of the skull of each species, and the three canals inside. As well as the primates, they also looked at 119 other mammals, from mouse to elephant, and gave each one a score from one to six, based on how swift or agile they were.

Canal size predicts agility

As predicted, they found that the canals of agile animals with fast, jerky movements like tarsiers (image below, left) are larger for their body size and more strongly curved. Slower species like lorises have relatively small and less curved canals.

Spoor’s data suggests that the size of the semicircular canals are an important adaptation to give fast-moving animals greater stability.

It explains why some primates can gracefully race through dense treetops at speeds where humans, with out relatively smaller canals, would embarrassingly collide with a branch. Just look at this amazing video from the Life of Mammals, of various lemurs (and their predators) moving through the trees.

This method can also be used forensically, to recreate the movement styles of extinct mammals. To prove this principle, Spoor looked at the canals of several species of extinct lemur, and found that their canals gave important clues about their behaviour.

Of the species he looked at, Palaeopropithecus (image above, right) had by far the smallest canals for its size. Accordingly, palaeontologists believed it was the lemur equivalent of a sloth; its hands and feet are curved for hanging from branches, and its wrists and ankles have lost the flexibility needed for effective walking.

Reference: Spoor, Garland, Krovitz, Ryan, Silcox & Walker. 2007. The primate semicircular canal system and locomotion. PNAS doi/10.1073/pnas.0704250104

Technorati Tags: semicircular canal, inner ear, balance, mammals, agility, animal locomotion, primates, science

Images: Top and bottom images from Alan Walker lab, Penn State, siamang by William H Calvin, loris by Sandilya Theuerkauf

Related stories on mammal evolution:

The evolution of animal personalities – they’re a fact of life
Orang-utan study suggests that upright walking may have started in the trees
Living optic fibres bypass the retina’s back-to-front structure
Death of dinosaurs did not lead to rise of modern mammals
Human cone cell lets mice see in new colours

Orang-utan study suggests that upright walking may have started in the trees

A common theory of human evolution says that after our ancestors descended from the trees, they went form walking on four legs to two. But a new study in orang-utans could overturn that theory, by suggesting that our ancestors evolved a bipedal walk while they were still in the trees.

Walking on two legs, or bipedalism, immediately sets us apart form other apes. It frees our arms for using tools and weapons and is a key part of our evolutionary success. Scientists have put forward a few theories to explain how our upright gait evolved, but the ‘savannah theory’ is by far the most prolific.

It’s nicely illustrated by this misleading image that has become a mainstay of popular culture. It suggests that our ancestors went from four legs to two via the four-legged knuckle-walking gait of gorillas and chimps. Dwindling forests eventually pushed them from knuckle-walking to a full upright posture. This stance is more efficient over long distances and allowed our ancestors to travel across open savannahs.

But this theory fails in the light of new fossils which push back the first appearance of bipedalism to a time before the forests thinned, and even before our ancestors split from those of chimpanzees. Very early hominins, including Lucy (Australopithecus afarensis) and Millennium Man (Orrorin) certainly ambled along on two legs, but they did so through woodland not plains.

Our arms provide a further clue. Even though our ancestors’ back legs quickly picked up adaptations for bipedalism, they steadfastly kept long, grasping arms, an adaptation more suited to moving through branches. To Susannah Thorpe at the University of Birmingham, these are signs that bipedalism evolved while our ancestors were still living in trees.

Two legs good, four legs bad?

But there is a snag – an adaptation must provide some sort of benefit. And, as many children painfully discover, it is hard to imagine how walking on two legs could benefit sometime in a tree.

But Thorpe has an answer to this too. She spent a year in the Sumatran jungle, studying the orang-utan – the only great ape to spend the majority of its life in the trees.

She carefully documented over 3,000 sightings of wild orang-utans moving through the treetops. On large sturdy branches, they walk on all fours (below right), and on medium-sized ones, they start to use their arms to support their weight.

But on the thinnest and most unstable branches, the apes use a posture that Thorpe calls ‘assisted bipedalism’ (below left). They grip multiple branches with their long, prehensile toes and use their arms to balance and transfer their weight. And unlike chimps which bend their knees while standing up, bipedal orang-utans keep their legs straight, just like humans do.

It’s a win-win posture – the hands provide extra safety, while the two-legged stance frees at least one hand to grab food or extra support. With it, the apes can venture onto the furthest and thinnest branches, which provides them with several advantages.

As Thorpe says, “Bipedalism is used to navigate the smallest branches where the tastiest fruits are, and also to reach further to help cross gaps between trees.” That saves them energy because they don’t have to circle around any gaps, and it saves their lives because they don’t have to descend to the ground. “The Sumatran tiger is down there licking its lips”, she said.

A new view of ape & human evolution

With these strong adaptive benefits, it becomes reasonable to suggest that bipedalism evolved among the branches. Based on this theory, Thorpe, along with Roger Holder and Robin Crompton from the University of Liverpool, have painted an intriguing new picture of ape evolution.

It begins in the same way as many others – with the rainforests of the Miocene epoch (24 to 5 million years ago) becoming increasingly patchy. For tree-dwelling apes, the gaps in the canopy started becoming too big to cross. But in Thorpe’s view, these ancestral apes were already using a bipedal stance, and different groups took it in separate directions.

The ancestors of orang-utans remained in the increasingly fragmented canopy and became specialised and restricted there. The ancestors of chimps and those of gorillas specialised in climbing up and down trees to make use of food both in the canopy and on the ground. The postures used in vertical climbing are actually very similar to those used in four-legged knuckle-walking and this became their walk of choice on the ground.

The ancestors of humans abandoned the trees altogether. They used the bipedal stance that served them well on thin branches to exploit the potential of the stable land environment. Over time, they brought in further adaptations for efficient walking, culminating in the human walking style that we now neglect by sitting at a computer all day.

Thorpe’s reconstruction is delightfully non-human-centric. It suggests that in the evolution of movement, we were conservatives who relied on a walk that had been around for millions of years. Chimps and gorillas with their fancy new knuckle-dragging gait were the true innovators.

Reference: Thorpe, Holder & Crompton. 2007. Origin of human bipedalism as an adaptation for locomotion on flexible branches. Science 316: 1328-1331.

Image: Black and white image from Science magazine.

Technorati Tags: human evolution, apes, orang-utans, bipedalism, walking on two legs,

Related stuff:
On ape and human evolution:
Chimps show that actions spoke louder than words in language evolution
Hidden ‘junk’ gene separates human brains from chimpanzees
Chimps have more adaptive genetic changes than humans

On the evolution of movement:
Salamander robot walks, swims and sheds light on evolutionary step from sea to land
Microraptor – the dinosaur that flew like a biplane

Spread the word: Digg this Del.icio.us Reddit Google Bookmarks Stumbleupon

Living optic fibres bypass the retina’s back-to-front structure

The human retina is back-to-front. Its silly structure means that light has to cross a tangle of nerves and blood vessels before it reaches the light sensors at the back. Now, scientists have found that the retina uses special cells called Muller cells that funnel light through the retina, in the style of living optic fibres.

If you were a designer tasked with creating a machine for collecting and processing light, the last thing you would come up with is the human eye. Darwin marvelled at the eye’s perfection, but in this, he was wrong. Aside from the many illusions that can fool it, our eyes have a major structural flaw.

In humans and other back-boned animals, the light-sensing cells of the eye lie at the back of the retina (see image right; courtesy of University of Michigan).

In front of these sensors lie several layers of nerve cells that carry their signals, and blood vessels that supply them with nutrients. The nerves join to the main optic nerve which passes through a hole in the centre of the retina and connects to the brain.

It’s a stupid, back-to-front design. Light has to pass through several layers of nerves, not to mention blood vessels, before it hits the retina itself. It’s a bit like designing a camera, and sticking the wiring in front of the lens.

Octopuses and squid have a very similar eye to ours, but theirs’ are much more sensibly structured. Their nerves and blood vessels connect to the light sensors from behind so that light can hit the sensor cells without having to negotiate an obstacle course. And because their retina doesn’t need a hole to accommodate the optic nerve, they have no blind spot.

In our own retinas, nerves and vessels are random in their spacing and irregular in their shape. The light that shines past them is reflected, scattered and refracted.

It’s amazing that our eye can see at all. But even though there is clearly no designer, evolution does a pretty good job instead. It has a remarkable capacity for making the best of a bad job. In the case of our eye, some of the obscuring cells act as living optic fibres, to funnel light onto the sensors it covers.

Muller cells – living optic fibres

Kristian Franke and colleagues from the Paul Flechsig Institute for Brain Research first noticed these fibres by shining light onto the retinas of guinea pigs. They looked at a cross-section near where the light sensors lay and saw a very regular pattern of bright spots. Clearly, some parts of the retina were transmitting light far better than others.

As they looked at further cross-sections throughout the retina, they realised that the bright spots were the endpoints of long tubes that stretched throughout the retina. Near the top, the tubes widened into funnels.

Franze identified these tubes as Muller cells. The brain cells aren’t nerves themselves, but are part of their supporting cast. They are long cylinders arranged in columns across the entire retina, and provide a route for light to pass through the tangled morass of nerves and blood vessels.

How they work

The Muller cells gather light at the top of the retina and channel it to the light sensors as a tight beam. Along the way, the light is barely reflected or scattered and little is lost when it finally reaches the light sensors, just like modern optic fibres.

Light enters the Muller cells at a shallow angle and is slowed down considerably by the cells’ high refractive index. When it hits the cells’ boundaries, it is almost completely reflected back along the tube.

Their funnel shape allows the Muller cells to gather and transmit as much light as possible. But as they narrow in the middle, they take up a very small amount of space and leave plenty of room for the blood vessels and nerves that the retina needs.

On average, each Muller cell serves a single cone cell and several rod cells. This one-to-one system ensures that the images that eventually hit the light sensors keep strong contrast, and are not distorted.

Evolution has given the vertebrate eye a remarkably ingenious solution to its ludicrous inverted retina. The eye may not be the perfect organ that Darwin thought, but new insights into its’ evolution still provides us with awe-inspiring surprises.

 

Reference: Franze, Grosche, Skatchkov, Schinkinger, Foja, Schild, Uckermann, Travis, Reichenbach & Guck. 2007. Muller cells are living optical fibers in the vertebrate retina. PNAS 104: 8287-8292.

Technorati Tags: retina, human+eye, eye+evolution, Muller+cells, optic+fibres, intelligent+design, science

• Digg this
• Del.icio.us
• Reddit

Chimps show that actions spoke louder than words in language evolution

Chimpanzees and bonobos use gestures more flexibly and adaptively than other forms of communication. These gestures, and not words, may have been the starting point for the evolution of human language.

Hollywood cavemen typically communicate with grunts and snorts, reflecting a belief that human language originated like this and slowly evolved into the rich and sophisticated tongues we use today.

But researchers from Emory University, Atlanta have found evidence that the origins of human language lie in gestures, not words. If they are right, then high-fives, V-signs and thumbs-ups could more closely reflect the beginnings of human language than conversations do.

The importance of gestures

All primates can communicate with each other through facial expressions, body postures and calls, but humans and apes are unique in their use of gestures. These go beyond simple postures or walking patterns – they are movements of the hand, limbs and feet, specifically directed at another individual.

We think of language as mainly spoken or written but gestures play an enormous, often overlooked role. After all, isn’t a speaker who waves their hands animatedly more engaging than one who stands motionless behind a podium?

And they are such an intrinsic part of the way we communicate that a blind speaker will make gestures normally to a blind audience, and babies use gestures long before they learn their first words.

To understand the role of gestures in the origins of human language, Amy PIllock and Frans de Wall decided to see how they are used by our closest relatives – the chimpanzee and the bonobo (the pygmy chimpanzee

How chimps and bonobos communicate

For one and a half years, they watched 34 chimps belonging to two separate groups, and 13 bonobos (right), again from two groups. Through painstaking analyses, they identified 31 different gestures and 18 different facial and vocal expressions.

The facial and vocal signals were very consistent between the two species. They were used in the same ways, and in very specific contexts, such as play, grooming or fights. For example, both chimps and bonobos scream when threatened or attacked, a gesture that is shared by many other primates.

This suggests that these signals are an evolutionarily ancient means of communication, and were probably used by the common ancestor that we shared with both chimps and bonobos.

Gestures on the other hand, were altogether more flexible. They were used in all sorts of situations and carried very different meanings depending on the context.

When a chimp stretches out an open hand (below), it can be asking for support during a fight or for a share of food during a meal. In the same way, a person raising his hand could be greeting a friend, surrendering, or answering a question in class.

Chimps and bonobos also differed considerably in their vocabulary of gestures, with each species having its own ‘gesture culture’. The two groups of bonobos even used slightly different sets of gestures to each other.

Gestures and the origins of human language

Pillock and de Waal believe that these studies strongly position gestures as the starting point for human language evolution. In chimps and bonobos, gestures are more adaptable and flexible than calls or facial expressions.

They are relatively disconnected from specific emotions and can be more easily controlled. Facial expressions can give away big clues about a person’s emotional state in all but the best poker faces, but gestures can be used subtly or even deceptively.

Their adaptability means that gestures can be used in many ways and are free to pick up a variety of symbolic meanings. They pick up cultural differences easily, as shown by the very different ‘gesture vocabularies’ used by chimps and bonobos.

Pillock’s and de Waal’s experiments also support other studies which suggest that the language of bonobos is more sophisticated than that of chimps. For a start, their gestures show greater cultural variations.

While bonobos combine them with calls and facial expressions less frequently than chimps do, but they also respond much more strongly to these joint signals. Pillock and de Waal believe that the bonobos could be using these multiple signals more deliberately than chimps to subtly change the meaning of a facial expression or vice versa.

The chimps on the other hand, may just be using joint signals to say the same message more loudly. Among the great apes, the bonobos may deserve the silver medal for their language skills.

Reference: Pollick & de Waal. 2007. Ape gestures and language evolution. PNAS 104: 8184-8189

Technorati Tags: chimpanzees, chimp+gestures, bonobo+gestures, language+evolution, animal+communication, origins+of+language, science

• Digg this
• Del.icio.us
• Reddit

Chimps have more adaptive genetic changes than humans

According to new research, chimpanzee genes have shown more adaptive changes than those of humans. The media widely reported the results as evidence that chimps are ‘more evolved’ than humans. But as I discuss here, these headlines are putting words into the researchers’ mouths.

Since the time when humans and chimps evolved from our common ancestor, our species appears to have come on by leaps and bounds. We walk on two legs, we speak using languages and while there is no doubt that chimps are intelligent, there is even less doubt that our brainpower outclasses theirs.

For years, scientists have assumed that our advanced abilities must be reflected in our genetics. After all, traits like intelligence and language give us great adaptive advantages. They should therefore be mirrored by similarly large changes in the human genome, compared to the chimp one.

Not so. Researchers at the University of Michigan sifted through the human and chimp genomes for signs of positive selection – the process where natural selection firmly embeds new mutations because of the advantages they provide. They found that the chimp genome contains 50% more positively-selected genes than the human one.

While earlier studies have compared individual human and chimp genes, this is the first to do a proper census. Margaret Bakewell and colleagues looked at almost 14,000 genes in both species.

The project was given a valuable push by the recent publication of the fully-sequenced rhesus macaque genome. The macaque – a type of monkey – is an evolutionary cousin of both humans and chimps, and provides a useful comparison.

If the team found a difference in the human and chimp genes, the macaque version can tell them which version is closest to the ancestral one. Previously, scientists had to make do with the mouse, a much more distantly related animal. The macaque’s presence gives the analysis greater accuracy.

Bakewell failed to find any noticeable differences in the function of positively-selected genes in humans and chimps. Both species even had similar proportions of positive changes among the genes that control the brain and nervous systems.

The reasons for this surprising result are unclear, but Bakewell feels that population sizes may hold the answer. For most of their evolution, chimpanzees have enjoyed a larger population size than humans have. It’s only recently that our numbers have ballooned to unfeasible proportions.

According to evolutionary theory, beneficial genetic changes are more quickly established in a population if it is larger. But in smaller groups, random genetic changes can trickle down through generations without being properly weeded out. This ‘genetic drift’ could explain why humans have fewer positively-selected genes than chimps do.

An alternative theory is that many of the human genetic changes that provide us with the greatest advantages may be relatively new developments. It is only recently in our history that we spread around the world from our origins in Africa, and as such, new genetic innovations may not have become established in the population as a whole.

A third theory, which I’m putting forward myself, is that the genetic changes responsible for our most human traits, may lie among stretches of DNA missed by this study. Recently, a study showed that one of the most important ‘genes’ in human evolution lies within our so-called junk DNA and controls the development of our brains. Clearly, we still have much to learn.

Nonetheless, the study helpfully shows that evolution is not necessarily about progress. It’s not an inexorable march toward some gleaming future. It’s about change, regardless of direction or result.

Somewhere along the line, the word ‘evolved’ started to gain a false value. It became an indicator of positive progress, so that claiming to be ‘more evolved’ than a peer is to claim superiority.

A huge number of newspapers and magazines reported this story under the headline of ‘Chimps more evolved than humans’. And while that may be technically reasonable, the inferences made were anything but.

The inherent values placed upon the phrase ‘more evolved’ clearly emerged in the reaction to the story. Some suggested that humans were obviously ‘less evolved’ given for reasons ranging from pollution to capitalism. Meanwhile, creationists and ID-supporters smelled blood in the water, and claimed that such as blatantly preposterous conclusion proved that evolution was nonsense.

Of course, no such conclusions were actually made by the study itself. In the light of the proper progress-free meaning of the word ‘evolution’, hese results are not preposterous, but fascinating. We should use them to drive a nail in the coffin of phrases like ‘evolutionary race’ or ‘more evolved’, at least in its value-laden non-scientific sense.

Reference: Bakewell, Shi & Zhang. 2007. More genes underwent positive selection in chimpanzee evolution than in human evolution. PNAS 104: 7489-7494.

Technorati Tags: chimp, chimpanzees, chimp genes, chimps humans evolved, chimps evolved, more evolved, science

• Digg this
• Del.icio.us
• Reddit

Death of dinosaurs did not lead to rise of modern mammals

New research has disproved the idea that the extinction of the dinosaurs allowed mammals the chance to take over the earth. Modern mammal groups only diversified sometime after the mass extinction. But if dinosaurs weren’t holding them back, what was?

While the mighty dinosaurs walked the earth, the ancestors of modern mammals were scurrying through the undergrowth beneath their feet, biding their time. Sixty-five million years ago, their opportunity came.

During a massive extinction event, the majority of life on earth including the entire dinosaur line (with the exception of the birds) went extinct. And like ambitious young graduates whose boss got the sack, the mammals took their ecological place.

They rapidly diversified into a variety of different forms, eventually giving rise to the four thousand plus species that exist today. Right?

Wrong.

This is the picture that has been painted by scientists, textbooks and popular culture for decades. But in the light of new evidence, it just doesn’t measure up.

Olaf Bininda-Emonds, Andy Purvis and a team of international scientists reconstructed the tree of life that unites almost all living species of mammals. Using both fossils and genetic evidence, they worked out how modern species split apart from common ancestors over the last several million years.

Their study turned up many surprises. For a start, they found that the seeds of modern mammal dynasties were planted much earlier than expected.

Almost twice the expected number of mammal groups were around to see the K/T boundary ­– the point in time where the dinosaurs and their peers went extinct. While the dinosaurs were still stomping about, the early mammals were busy exploiting smaller ecological niches.

But after the K/T boundary, the researchers found no sign of the expected rapid expansion of mammal lineages. As it turns out, mammals – or our forefathers at the very least – were not impatient go-getters ready to spring into action when opportunity knocked. In evolutionary terms, they were slackers.

The fuse that led up to the explosion of modern mammal groups was clearly longer than we thought. And Bininda-Emonds’s work suggests a reason for that too.

As it happens, some mammal groups did diversify quickly after the dinosaurs’ coup de grace, giving rise to species like Andrewsarchus (right), a ferocious hooved predator. But these animals too eventually went extinct. Today, they have no living descendants.

These temporarily dominant groups, like the dinosaurs before them, may have kept our own ancestors in the evolutionary shade. As Purvis explains, “For the first 10 or 15 million years after the dinosaurs were wiped out, present day mammals kept a very low profile, while these other types of mammals were running the show.”

Perhaps they adapted to a still-changing climate only to be wiped out mere geological moments later. The forefathers of today’s mammals could have fared better because they played it slow and steady, and waited until conditions settled down before diversifying.

Reference: Bininda-Emonds, Cardillo, Jones, MacPhee, Beck, Grenyer, Price, Vos, Gittleman & Purvis. 2007. The delayed rise of present-day mammals. Nature 446: 507-512.

 

Related posts on dinosaurs and mammal evolution:
How many types of dinosaur were there?
Microraptor – the dinosaur that flew like a biplane
Tracks provide evidence of swimming dinosaurs
Bone-crushing super-wolf went extinct during last Ice Age
Orang-utan study suggests that upright walking may have started in the trees
Human cone cell lets mice see in new colours

Technorati Tags: prehistoric life, palaeontology, early mammals, prehistoric mammals, mammal evolution, death of dinosaurs, dinosaur extinction, K/T+boundary, science

• Digg this
• Del.icio.us
• Reddit

Human cone cell lets mice see in new colours

While humans have three types of colour-detecting cells, mice and most other mammals have just two. But when a group of scientists gave mice the human gene for a third colour detector, they were able to detect colours that no mouse has ever seen before. This profound response to a simple genetic change may show us how our own vision evolved.

Evolution is almost synonymous with small, gradual changes, and for good reason. We might expect that large changes to an animal’s genetic code, and therefore to its body plan, simply wouldn’t work. It would be like shoving an extra cog into a finely-tuned machine and expecting it to fit in – the more likely outcome is a malfunctioning mess.

But that’s not always the case, at least not for the evolution of the human eye. To Darwin, the eye was so perfect that it made him cast serious doubts over his theory of evolution.

But new research shows that the eye and its connections to the brain are surprisingly flexible, and can incorporate major evolutionary changes with ease.

In our retinas, cone cells are responsible for giving us colour vision. Most mammals have just two types, one that is sensitive to short violet-ish wavelengths of light (S cones), and another that responds strongly to medium greenish-blue wavelengths (M cones).

But somewhere in our history, humans and many other primates picked up a third cone sensitive to longer wavelengths (L cones), that allows us to see colours near the red end of the spectrum.

You might expect that adding another type of cone cell into the eye would be a very large step, requiring substantial (and gradual) changes in the wiring of both the retina and the brain. But Gerald Jacobs from the University of California has shown that it’s as easy as installing new software into your computer.

Together with Jeremy Nathans from Johns Hopkins Medical School, Jacobs genetically engineered a strain of mice that had human L cones in addition to its medium and short-wavelength ones.

Using a technique called electroretinography, they confirmed that these added cones were in full working order and were sending electrical outputs to the brain. They were clearly providing visual signals, but did this translate into any meaningful information in their brains?

Nathans and Jacobs set the mice a challenge to test their new retinal powers. They were shown three panels lit with coloured lights and had to pick out the one that was lit differently.

The normal mice failed to tell the difference between greenish-blue and yellowy-orange lights. They only chose correctly about a third of the time – the success rate you’d expect from random guesswork.

But the triple-coned mice passed with flying colours, so to speak. After lengthy training, they picked the odd panel out up to 80% of the time. Their genetic change had clearly been smoothly slotted in to their nervous system. They were seeing in three colours.

The success of Nathans’s experiment is testament to the tremendous flexibility of a mammal’s nervous system. And it gives us a tantalising glimpse into how modern primate vision evolved.

At some point in our evolutionary past, one of our ancestors was born with a mutation that gave it a third and slightly different type of cone cell. This change would have brought it an instantaneous competitive edge over its two-coned peers.

Having three types of cones, rather than two, greatly expands the range of light that an animal can detect, and gives it a much broader colour palette. Such an animal would have gained a deeper appreciation of its surroundings than its peers – its eyes would quite literally have been opened to new possibilities.

Some scientists believes that the key advantage lay in being able to discern unripe green fruit from ripe ones that are typically red or orange. The bright colours of fruit may even have co-evolved with the advent of three-coned primate vision, to take advantage of these new seed-dispersing agents.

Over future generations, these new visual powers would have been honed by further genetic changes, but it is highly likely that the initial genetic jump-start would have spread like evolutionary wildfire.

The same may even apply to other senses, like taste and smell, with new genetic changes caused profound effects by adding new receptors and expanding an animal’s sensory range.

Reference: Jacobs, Williams, Cahill & Nathans. 2006. Emergence of novel colour vision in mice engineered to express a human cone photopigment. Science 315: 1723-1725.

Technorati Tags: colour vision, colours, sight, eye evolution, vision, evolution, eyes, science

• Digg this
• Del.icio.us
• Reddit

Opinion: Not so unique – the chimpanzee Stone Age, and our place among intelligent animals

This is the 50^th article for this blog. I’ve been writing for it for over six months now, and I pleasantly surprised that I’m still finding the enthusiasm to write for it regularly, and that people seem to be reading it.

This special article considers new evidence for the origins of chimpanzee tool use. It is the third piece of research I’ve seen in the last few months which shows that other animals share an ability previously thought to be the sole province of humans. In this article, I consider why these discoveries are now coming to light, and what they mean for us.

In the Ivory Coast, a small stream called Audrenisrou winds its way through the lowland rainforest of the Tai National Park. On the floodplain of this stream, at a site called Nuolo, lie several stones that seem unassuming at first glance. But to the trained eye, they are a window to the past.

Their shape is different to other stones that have been worn away by natural erosion. They have been flaked in systematic ways and many are flattened and sharp. Clearly, they were shaped by hand for a purpose – they are tools.

Their creators were not humans, but close relatives who lived in these rainforests thousands of years ago – the ancestors of modern chimpanzees.

Nuolo: humans or chimps?

The Nuolo stones were uncovered by Julio Mercader form the University of Calgary, Christophe Boesch from the Max Planck Institute of Evolutionary Anthropology, and their colleagues. They are a magnificent archaeological find – the first ever evidence of prehistoric ape behaviour anywhere in the world.

Humans have a rich prehistoric past, informed by key archaeological finds like the Olduwan sites. These findings provide us with a window into the past, showing us how our ancestors developed the tools that continue to serve us well today. For chimps, no such sites have been found, until now.

The evidence that the Nuolo specimens were created by chimps is compelling. The density of stone pieces in the site, the preferred types of rocks, the length of the stone flakes and the patterns of wear closely mirror those of modern chimp tools.

They also carry the evidence of their past uses, as hammers and anvils for cracking nuts. Their crevices contain granules of starch that clearly came from nuts. Mercador and Boesch even managed to narrow the granules’ origins down to three possible species, all of which are currently cracked and eaten by today’s chimps. In contrast, the team found scant remains of tubers and legumes, the main food source of forest-dwelling humans.

This suggests that prehistoric humans who also, over time, visited the river-side site were not the creators of the Nuolo tools. But Mercador and Boesch found even stronger evidence.

Human hammers usually weight less than 400g, and even our ancestors’ anvils weighed no more than a kilogram. The far more powerful chimp with its larger hand can wield a tool many times heavier, anywhere from one to nine kilograms in weight.

Mercador and Boesch found that the stone tools at Nuolo most likely weighed about 2 kilograms, far too heavy heavier for a human but well within the limits of even a weak chimpanzee.

A chimpanzee Stone Age

Together, this evidence paints a remarkable picture of a chimpanzee Stone Age, when ancient chimps were clearly cracking nuts in the same way they do now, over four millennia ago.

Chimpanzees are highly advanced tool users. But some critics have sold short their abilities, claiming that they learned the use of tools by, for lack of a better word, apeing nearby humans.

The Nuolo finds puts paid to that suggestion. The tools predated the advent of farming in the rainforest by some time. Nuolo also lacks evidence of any of the other tools used by humans to grind and pound starchy tubers.

Among chimpanzees, nut-cracking is clearly a cultural tradition, passed down over time through over 200 generations of chimps. Humans and chimps either developed this technology independently, or they inherited it from a common ancestor who had already begun to use tools.

We’re not so unique after all

With studies like this, the list of attributes that are unique to humans seems to be getting smaller all the time. In just the last few months, scientists have found that chimpanzees hunt with spears, jays (below) can plan for the future, and even the long-dead dinosaur Bambiraptor, gripped prey with opposable fingers.

But as we start to come down from our pedestal, we should not mourn the loss of our position, but rejoice in our connectedness with the rest of the living world. These discoveries emphasise our position at the end of a continuous evolutionary spectrum, rather than atop a looming precipice.

The outdated view that we have been awarded special dominion over other life should be replaced by a humbler view, where our position of biological authority is tempered with respect.

Why has it taken so long for such findings to come to light? Centuries ago, anthropomorphism was commonplace and these experiments would have seemed like pointing out the obvious. But of late, biology has taken a more reductionist turn and signs of potential animal behaviour are scrutinised under the harshest and most sceptical light.

In many cases, this quite rightly avoids the false conclusions based on flimsy and anecdotal evidence. But while scientists have taken great care to ensure that their interpretations are not biased towards human perspectives, the same cannot always be said the design of the experiments themselves.

Looking for intelligence

One of the most significant problems with studying animal intelligence is that many species experience and react to the world in completely different ways to us. For example, to pass the classic test for self-awareness, an animal must show that it recognises itself in a mirror, by examining a mark previously made on its face (see right).

Gorillas and dogs tend to fail the mirror test, but not because they are mentally less advanced than successful examinees like elephants or chimps. Gorillas view direct eye contact is a sign of aggression and tend to avoid it, while dogs rely on smell as their primary sense, rather than sight.

Simply put, can we truly claim to understand the limits of another animal’s intelligence when we know so comparatively little about their behaviour or perceptions? Cleverly designed experiments may bring us closer to an answer, but sadly, we may never get the opportunity to conduct them.

Save ourselves, the most intelligent animals on the planet – the great apes, elephants, dolphins and whales – are mostly endangered, with many species facing a very real threat of extinction. Chimpanzees, like those in the Tai National Park are under threat from the loss of their habitat, and the illegal bushmeat trade.

A massive amount of evidence now paints these, our closest cousins, as sophisticated animals with their own culture. Imagine how tragic it would be if they died out for good, leaving only a set of shaped stones as the only lasting signs of their intelligence.

Reference: Mercader, Barton, Gillespie, Harris, Kuhn, Tyler & Boesch. 2007. 4,300-Year-old chimpanzee sites and the origins of percussive stone technology. PNAS 104: 3043-3048.

Technorati Tags: chimpanzees, chimps, chimp tools, animals tools, animal intelligence,

• Digg this
• Del.icio.us
• Reddit

Bats: internal compasses and record-breaking tongues

Bats are masterfully adapted to life in the night skies. Now, two new studies demonstrate previously unknown abilities in bat species – an inbuilt magnetic compass, and a tongue so big it has to be stored in the rib cage.

If you were a biologist looking for astounding innovations in nature, you could do much worse than to study bats. Bats are like showcases of nature’s ingenuity, possessing a massive variety of incredible adaptations that allow them to exploit the skies of the night.

They are the only mammal group capable of true flight and are one of only four groups of animals to have ever evolved the ability. As a result, they have spread across the globe with tremendous success. Today, one in every five species of mammal is a bat.

They are most famous for their incredible echo-location. By listening for the echoes of sound waves rebounding off solid objects, bats have been finding their way in the dark using a radar that humans have only managed to duplicate millions of years later.

Magnetism

Echolocation is a short-range skill. Over longer distances, the bats’ ability to control their signals and perceive their echoes weakens, and other navigational skills must be used.

Until now, it was unclear what these might be. Richard Holland and colleagues from Princeton University have changed all that by showing that a large North American species, the big brown bat (Eptesicus fuscus), finds its way home by using the Earth’s magnetic field as a compass.

Holland took several big brown bats 20km away from their roosts and tracked their way home with radio telemetry. Before they were released, the bats were acclimatised at sunset to a magnetic field that was rotated to face either east or west.

For 5km, the group exposed to the easterly oriented field flew east, and those exposed to the westerly oriented field flew west. They were clearly using a magnetic compass which had been calibrated during sunset.

After initially losing their bearings, many of the confused bats corrected themselves and found the right way home. Holland believes that they recognised that the direction they were flying in did not match their magnetic map, possibly through other cues such as the position of the stars.

Other species including turtles and homing pigeons are known to navigate with a magnetic sense, but this is the first time the ability has been shown in bats. It adds to the already impressive array of senses that these animals possess.

A wicked tongue

Bats are also known for the variety of different food sources they exploit. Some species specialise in snatching spiders from their webs using their peerless radar, others take fish from lakes and perhaps the most famous representative drinks the blood of other mammals.

And many species make a living by drinking the nectar of the bountiful flowers found throughout the tropics, like leather-winged equivalents of hummingbirds.

Hummingbirds have evolved highly specialised relationships with flowers, so that some flowers are only accessible to a single species with the correctly shaped bill. Until recently, no such partnerships were seen between flowers and bats, mainly because the bat’s soft facial tissues are less easily moulded by natural selection than the bird’s hard bill.

The tube-lipped nectar bat (Anoura fistula) from the Andean forests of Ecuador is a striking exception (see left; photo by Nathan Muchhala).

These cloud forests are home to a plant called Centropogon nigricans that has flowers 8-9cm long. No ordinary bat can feed from these.

Nathan Muchhala from the University of Miami discovered that the tube-lipped nectar bat manages it with a tongue that is 50% longer than its body. In terms of relative size, its tongue is second only to the chameleon’s.

But where does a 5.5cm long bat store an 8.5cm long tongue? In most mammals, the base of the tongue is attached at the back of the mouth. But in the nectar bat, it is stored in its rib cage and its base lies between its heart and its sternum. Muchhala believes that the bat’s tongue and the Centropogon’s flower co-evolved to extreme lengths over time.

He captured various local bats over four months and found pollen from Centropogon nigricans only on the fur of the tube-lipped nectar bats and not related species.

Due to their exclusive relationship, the bat gets a permanently reserved dining spot and the flower gets a dedicated pollinating service.

Holland, Thorup, Vonhof, Cochran & Wikelski. 2006. Nature 444: 702.

Muchhala. 2006. Nature 444: 701.

Technorati Tags: bats, bat behaviour, magnetic sense, animal navigation, big brown bat, nectar bat, science

• Digg this
• Del.icio.us
• Reddit