Short lives, short size – why are pygmies small?

For decades, anthropologists have debated over why pygmies have evolved to be short. Amid theories about their jungle homes and lack of food, new research suggests that we have been looking at the problem from the wrong angle. The diminutive stature of pygmies is not a direct adaptation to their environment, but the side-effect of an evolutionary push to start having children earlier.

Andrea Migliano at the University of Cambridge suggests that pygmies have opted for a ‘live fast, die short’ strategy. Their short lives gives them very limited time as potential parents, and they have adapted by becoming sexually mature at a young age. That puts a brake on their pubescent growth spurts, leaving them with shorter adult heights.

The social life of our extinct relatives

One of our extinct evolutionary cousins, Paranthropus robustus, may have walked like a man but it socialised like a gorilla. Using only fossils, UCL scientists have found that P.robustus males were much larger than females, competed fiercely for mates and led risky lives under heavy threat from predators.

I wrote an article about the cool new finding for Nature Network. Here’s the opening and you can read the full article here.

A single fossil can tell you about the shape, diet and movements of an extinct animal but with enough specimens, you can reconstruct their social lives too.

Charles Lockwood of University College London used an unusually large collection of fossils to peer back in time at the social structures of one of our closest extinct relatives, Paranthropus robustus, which inhabited southern Africa between 1.2 million and 2 million years ago.

Why are women better at food shopping than men?

Men do better than women at most tests of spatial awareness, but not all. A new study set in a farmer’s market shows that women outperform men at remembering the locations of food, particularly the most calorific ones.

When men and women do the grocery run, their evolutionary histories play out among the aisles of food in subtle ways. Women are more likely to remember where things are; men are better at plotting efficient paths through the smorgasbord of choice. These different abilities are the result of evolutionary adaptations that took place when we were still hunting and gathering.

The evolution of sex differences

The brains of men and women are clearly different, and rarely more so than in the realms of spatial awareness. In most tests of spatial ability, men routinely outperform women. But to Irvin Silverman and Marion Eals, this crude assertion crumbled under an evolutionary spotlight.

In 1992, the duo noted that our mental abilities were not created from a vacuum – they evolved to allow us to cope with different adaptive challenges. And for the men and women of our dim evolutionary past, these challenges were very different.

Back in the day, when we were still living as hunter-gatherers, men did most of the hunting while women excelled at gathering. And these jobs required very different spatial skills. Hunters, for example, needed to chase their prey over unfamiliar and winding routes; once they had killed, they needed to work out the quickest route home.

Resistance to an extinct virus makes us more vulnerable to HIV

Immunity to viral infections sounds like a good thing, but it can come at a price. Millions of years ago, we evolved resistance to a virus that plagued other primates. Today, that virus is extinct, but our resistance to it may be making us more vulnerable to the present threat of HIV.

Many extinct viruses are not completely gone. Some members of a group called retroviruses insinuated themselves into our DNA and became a part of our genetic code.

Indeed, a large proportion of the genomes of all primates consists of the embedded remnants of ancient viruses. Looking at these remnants is like genetic archaeology, and it can tell us about infections both past and present.

Viral hitchhikers

When retroviruses (such as HIV, right) infect a cell, they insert their own DNA into their host’s genome, using it as a base of operations. From there, the virus can pop out again and make new copies of itself, re-infect its host or move on to new cells.

If it manages to infect an egg or sperm cell, the virus could pass onto the next generation. Hidden inside the embryo’s DNA, it becomes replicated trillions of times over and ends up in every single one of the new individual’s cells.

These hitchhikers are called ‘endogenous retroviruses’. While they could pop out at any time, they quickly gain mutations in their DNA that knocks out their ability to infect. Unable to move on, they become as much a part of the host’s DNA as its own genes.

In 2005, a group of scientists led by Evan Eichler compared endogenous retroviruses in different primates and found startling differences. In particular, chimps and gorillas have over a hundred copies of the virus PtERV1 (or Pan troglodytes endogenous retrovirus in full). Our DNA has none at all, and this is one of the largest differences between our genome and that of chimps.

Our ancestors shared a similar geographical range to the ancestors of these apes, and would have encountered the same viruses, including PtERV1. And yet, we were spared from infection, while the apes were not. Why?

Protecting against an ancient virus

Shari Kaiser and colleagues from the University of Washington and the Fred Hutchinson Cancer Research Center believed that the answer lies in a protein called TRIM5α that defends us from retroviruses. It latches onto the outer coat of incoming viruses, and tells other proteins to dismantle or destroy them.

Other primates have their own versions of TRIM5α that protect against a different range of viruses, and the protein has evolved dramatically in different primate lineages. Kaiser believed that our version of TRIM5α protected us from PtERV1, while that of other apes did not. To test her idea, all she had to do was to resurrect a dead virus.

Obviously, PtERV1 is long extinct, but its remnants exist inside the genomes of chimps. Kaiser compared dozens of these remnants and by identifying common elements, she worked out the ancestral sequence of the virus.

She created a small part of PtERV1 and fused it with bits of a modern virus, MLV, to create a fully-functioning hybrid. To nullify any potential for spread beyond the lab, she crippled the virus so that it could infect once and only once.

The reconstructed virus successfully infected mammal cells in a lab, but not when human TRIM5α was around. The guardian protein demolished the virus’s infectivity, reducing it by more than 100 times. As Kaiser predicted, our genomes are free of PtERV1 because TRIM5α killed it before it could reach our DNA.

Resist one virus, succumb to another

But this protection carries a price – it makes us vulnerable to HIV. Over the course of primate evolution, humans made an important change in the amino acid sequence of TRIM5α that allowed the protein to fight off PtERV1. When Kaiser changed the protein back to its original form, she found that it gained the ability to fight off HIV, but lost its resistance to PtERV1.

In fact, Kaiser found that no primate species has a version of TRIM5α capable of fighting off both viruses at the same time. We are resistant to ptERV1 and vulnerable to HIV, but chimps, gorillas, baboons and rhesus macaques show the reverse strengths and weaknesses.

When it comes to retrovirus immunity, there is no win-win situation. Having defeated one enemy, we have unwittingly made ourselves more vulnerable to another.

Reference: Kaiser, Malik & Emerman. 2007. Restriction of an extinct retrovirus by the human TRIM5a antiviral protein. Science 316:1756 – 1758.

Technorati Tags: viruses, endogenous retroviruses, HIV, PtERV1, primates, chimps, infections, immune system, science

Related posts on viruses and virus evolution:
The upside of herpes – when one infection protects against another
Viruses evolve to be more infectious in a well-connected population
Round peg, square hole – why our bird flu drugs are a fluke

Related posts on new medical discoveries:
Drugs that work against each other could fight resistant germs
The secret of drug-resistant bubonic plague
Neutralising anthrax – moving closer to a cure

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

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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

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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

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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

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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,

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The heavy cost of having children

Menopause is an evolutionary mystery – what benefit could there be in losing the ability to have children? New research shows that mothers who have many children pay a steep physical price, and this may help to explain the current falling birth rates around the world.

While philosophers and poets muse on the meaning of life, natural selection casts a dispassionate eye on the whole affair. From the viewpoint of evolution, there is only one thing that matters – that we survive long enough to pass our genes on to the next generation, as many times as possible. And from the viewpoint of evolution, we are not doing a very good job.

Birth rates in several countries around the world – the UK, Japan, China – are falling dramatically. Women are having fewer children and they are having them later, close to the end of their fertile period.

But the fact that women undergo menopause at all seems strange, and the reasons for this reproductive expiry date has long puzzled biologists. There doesn’t seem to be any obvious benefit to ending a woman’s child-bearing potential with many years or decades to spare.Nor is menopause a symptom of our healthy modern lives – even in traditional societies, women often survived long past this point.

The favoured idea is that women retire early from child-bearing for the same reasons that athletes retire from their sports at a young age – their bodies cannot handle the strain. Childbirth is a taxing process for a woman and at some point, it becomes too risky for mother and child.

Scientists have suggested that menopause is an evolutionary respite from the burdens of having children. Now, Dustin Penn at the Austrian Academy of Science and Ken Smith from the University of Utah have found compelling evidence to support this idea.

They looked at comprehensive records on over 21,000 couples living in Utah in the 19^th century. This time an place in history was not an easy one. Families migrated across the Western frontier, facing the hardships of uncertain food supplies, poor medical care and unknown dangers. In spite of this, families were very large, with many couples having 10 children or more.

Penn and Smith found that both parents pay a physical price for having children, but mothers particularly so. The more children a woman had, the lower her chances of survival became, and this extra risk lasted well into later, post-menopausal life.

The children’s health suffered too as their mothers went through more pregnancies. Those with more siblings were less likely to make it to their eighteenth birthdays, and the youngest proved to be the most vulnerable.

The duo reasoned that having more children gave mothers less time to recover from the many physical difficulties of pregnancy, such as nutritional deficiency and weakened immune systems, and the extra burdens of birth and lactation.

These burdens are even higher if women have to raise chronically unhealthy children, as the youngest ones in large families often were. Even at the cellular level, they tend to show the signs of stress and damage.

And the actual effect may even be greater than Penn and Smith found since many of these Utah settlers were Mormons and would have received considerable community help in raising their children.

A halt in the ability to have more children would allow women to focus on their existing families. Penn and Smith found that children were 78% more likely to die before the age of 18 if they lost their mothers first.

Even across just two generations, this small survival difference meant that women who died early left behind about three fewer grandchildren than those who survived to care for their kids.

Together, Penn and Smith’s results provide strong evidence that menopause evolved to allow older women to avoid the high cost of giving birth to more children, and concentrate on their existing ones. But it also explains why women tend to be choosier about their partners than men are. Childbirth carries greater costs for them than for men, and they have more to lose by making a bad choice.

This could explain why women are drawn to indicators of resource and future investment, while men are more likely to look for youth and waist-to-hip ratio – signs of reproductive potential and the ability to tolerate the stresses of childbirth.

The high cost of childbirth may also go some way to explain the modern decline in fertility. Low birth rates are more commonly found in countries where the sexes are relatively equal, and where women enjoy independence and greater opportunities for education. Given these opportunities, it seems that women prefer to have smaller families, perhaps instinctively to reduce the costs of reproduction.

Reference: Penn & Smith. 2006. PNAS 104: 553-558.

Technorati Tags: menopause, children, costs children, fertility, science

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Brain parasite drives human culture

The brain parasite, Toxoplasma gondii, is spread by cats and affects a huge proportion of the world’s population. It’s effects on our behaviour make it a potent driving force of human culture.

We like to think that we are masters of our own fates. The thought that others might be instead controlling our actions makes us uneasy. We rail against nanny states, we react badly to media hype and we are appalled at the idea of brainwashing.

But words and images are not the only things that can affect our brains and thoughts. Other animals – parasites – can do this too.

Now, Kevin Lafferty from the University of California, Santa Barbara, has found startling evidence that a common brain parasite, Toxoplasma gondii, could be influencing human culture across the globe.

Toxoplasma gondii is a single-celled brain parasite spread by cats. Our feline companions are its preferred home and only there can it mature and reproduce. So like most parasites, T.gondii has a complex life cycle designed to get it into its final host.

If it finds itself in another animal, it travels to the brain and changes the host’s behaviour to maximise its chances of ending up in a cat. For rodents, this means being eaten and infected individuals are less fearful of cats and more active, making them easier prey.

Humans can also contract the parasite, through contact with soil contaminated by the faeces of carriers or through eating infected meat. But since cats are very unlikely to eat humans, in our bodies, T.gondii reaches a cul-de-sac. Still, there is nothing to stop the parasite, evolutionarily speaking, from trying out the strategies that work so well in other hosts.

In rare cases, T.gondii infection causes a disease called toxoplasmosis that produces mild flu-like symptoms and only really threatens foetuses and those with weak immune systems. In most instances, the parasite acts more subtly.

Carriers tend to show long-term personality changes. Women tend to be more intelligent, affectionate, social and more likely to stick to rules. Men on the other hand tend to be less intelligent, but are more loyal, frugal and mild-tempered. The one trait that carriers of both genders share is a higher level of neuroticism – they are more prone to guilt, self-doubt and insecurity.

In individuals cases, these effects may seem quirky or even charming but across populations, they can have a global power. T.gondii infection is extremely common and rates vary greatly from country to country.

While only 7% of Brits carry the parasite, a much larger 67% of Brazilians are infected. Given that the parasite alters behaviour, infection on this scale could lead to sizeable differences in the general personalities of people of different nationalities. This is exactly what Lafferty found.

Neuroticism is one of the most widely-studied of all psychological traits and Lafferty found that levels in different countries correlated well with the levels of T.gondii infection. The parasites’ presence was also related to aspects of culture associated with neuroticism.

Countries where infection was common were more likely to have ‘masculine sex roles’, characterized by greater differences between the sexes and their part in society and a stronger focus on work, ambition and money rather than people and relationships. Strongly infected societies were also more likely to avoid risk and embrace strict rules and regulations.

Obviously, different countries are also not just uniform populations, and increasing rates of migration mean that many countries are very ethnically and culturally mixed. However, this works in favour of Lafferty’s theory as any mixing would serve to mask the link between infection and culture. If anything, the link is stronger than seen in this study.

It would be imprudent to suggest that T.gondii is the major driver of human culture. It is just one of a number of influences that include genes, our physical environment and our histories. And Lafferty himself is quick to point out caveats to his own results.

For a start, they do not imply that the parasite is causing these personality types; it could be that people with these traits are more likely to become infected. To establish the true direction of causality, Lafferty will need to find out how the parasite manipulates the mind. The general idea is that infection alters levels of the immune system’s communication chemicals – the cytokines – which in turn alter levels of neurotransmitters like dopamine. But the details remain a mystery.

Nonetheless, the results are striking and they suggest that climate could have a larger effect on culture than previously thought. Toxoplasma gondii’s eggs live longer in humid, low regions so variations in climate could influence the global distribution of cultural traits. Perhaps, this could explain why men and women perform more distinct roles in society in countries in warmer climates. Other factors can also affect the risk of infection, including cat ownership and national cuisines that include undercooked meat.

We like to think of culture as something governed by the collective actions of free-thinking and free-acting humans. But Lafferty’s analysis shows us that if environmental factors like parasites can affect our thoughts and actions, no matter how subtly, they can have a strong effect on national cultures.

In many cases, these effects could be much stronger than the agents that we normally believe to drive cultural trends. After all, more people around the world are infected with Toxoplasma than are connected to the internet.

Reference: Lafferty. 2006. Proc Roy Soc B 273: 2749 – 2755.

Technorati Tags: brain parasite, Toxoplasma, parasites, parasite culture, human culture, science

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Non-coding DNA drove human brain evolution by making nerve cells stickier

Most of our genome is made up of the poorly named ‘junk DNA’. New research shows that these sequences may have been vital in the evolution of human brains, by allowing our neurons to make better contacts with each other.

Two months ago, a group of scientists found that the gene that has evolved fastest since our evolutionary split from chimpanzees is found in our so-called ‘junk DNA’. DNA is a code that tells our cells how to build their molecular workforce – proteins.

But the vast majority of our DNA sequence is never translated into proteins. While some considered this ‘junk’ DNA to be meaningless, recent research has shown that it makes important contributions to our most human of organs – our brains.

Now, Shyam Prabhakar and James Noonan at the Lawrence Berkeley National Laboratory have found further proof of the link between non-coding DNA and our mental evolution.

They studied over 110,000 stretches of DNA called ‘conserved non-coding sequences’ (CNSs), that are largely similar in a wide variety of animals. Of these sequences, 992 showed large numbers of changes that were specific to humans.

This number is much higher than would be expected if these DNA regions were drifting aimlessly in the evolutionary river. Their frequency is the mark of natural selection – these sequences must have changed for a reason.

To discover what this reason might have been, Prabhakar and Noonan looked at which genes these CNSs were in, and what they do in the body. They found that a large proportion of the genes in question were involved in the adhesion of neurons (nerve cells).

These genes are vital for the growth and development of our brains and allow neurons to make connections with each other, and with their surrounding framework of supportive cells.

The duo found a similar number of CNSs with chimpanzee-specific changes and many of these were also involved in nerve cell adhesion. But there was hardly any overlap between the chimp-specific and human-specific sequences.

Both lineages have developed nerve cells that make better contacts with each other, but have done so in separate ways using different genes.

It is possible that human and chimp brains have evolved different mental abilities to satisfy different evolutionary pressures. Identifying the precise role of the human-specific CNSs will help to test this possibility and it is the next big challenge facing Prabhakar and Noonan.

In the meantime, this research once again shows that non-coding DNA, far from being useless junk, was vitally important for the evolution of the human brain and its many unique abilities. Subtle changes in these sequences separate us from even our closest animal relatives.

Prabhakar, Noonan, Paabo & Rubin. 2006. Science 314: 786.
Technorati Tags: brain evolution, human evolution, junk DNA, non coding DNA, chimpanzees, science

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