Tiny molecules drove the evolution of the vertebrates

Blogging on Peer-Reviewed ResearchThe spinal column that runs down your back is an identity badge that signifies your membership among the vertebrates – animals with backbones. Vertebrates have arguably the most complex bodies and genomes of any animal group and certainly, our lineage has come a long way from its last common ancestor.

TigerThe closest evolutionary cousins of the vertebrates are simple aquatic creatures such as the jawless lancelets and the sac-like, immobile sea squirts. How did these simple body plans diversify into the vast array of sophisticated forms wielded by today’s fish, amphibians, reptiles and mammals?

Gene number

Many scientists have suggested that the answer lies in the number of our genes. At three different points, the vertebrate genome (its full suite of genes) experienced a massive jump in size as huge chunks of genes – possibly the entire lot – were duplicated. The first of these coincided with the origins of the group itself and the second happened alongside the rise of the first jawed fish, setting them and their descendants aside from more ancient jawless forms like the lampreys.

So far, there seems to be a tidy connection between gene number and complexity, but the third round of duplication is a bit of a stumbling block. It happened at some point during the evolution of the bony fishes and while this group proceeded to radiate into a multitude of different shapes, their basic body plan stayed essentially the same. No big jump in complexity there.

Indeed, as the full genome sequences of more and more species are revealed, it’s becoming clear that the basic genetic toolkit that controls the development of animal bodies is remarkably consistent across the kingdom. Even the genome of a sea anemone, one of the simplest and most ancient animals on Earth, is strikingly similar to that of vertebrates.

In this light, it’s looking increasingly unlikely that the advent of new genes can account for the large rise in vertebrate complexity. Now, Alysha Heimberg and colleagues from Dartmouth College have proposed a new theory, centred around tiny molecules called microRNAs.

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Earliest bat shows flight developed before echolocation

Blogging on Peer-Reviewed ResearchTheir heads and bodies of bats have amassed an extraordinary array of adaptations that have make them lords of the night sky. Today, the thousand-plus types of bats make up a fifth of living mammal species. Richard Dawkins once described the evolution of bats as “one of the most enthralling stories in all natural history” and as of this week, the story has a clearer beginning.

OnychonycterisThe success of bats hinges on two key abilities: their mastery of flight, a feat matched only by birds and insects; and echolocation, the ability to navigate their way through pitch-blackness by timing the reflections of high-pitched squeaks. For evolutionary scientists, the big question has always been: which came first?

The ‘clawed bat’

Until now, fossil bats haven’t provided any clues for all of them show signs of both echolocation and flight. But a stunning new fossil, discovered by Nancy Simmons from the American Museum of Natural History is an exception and it provides a categorical answer to the long-running debate – the earliest bats could fly but could not echolocate.

The new creature hails from the Green River in Wyoming and is known as Onychonycteris, meaning “clawed bat”. Its fossils date back to about 52.5 million years ago and by comparing it to other prehistoric bats, Simmons found that it is the most ancient member of this lineage so far discovered. It acts as a ‘missing link’ in bat evolution, much like the famous Archaeopteryx hinted that birds may have evolved from dinosaurs.

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Malawi cichlids – how aggressive males create diversity

Blogging on Peer-Reviewed ResearchCertain groups of animals show a remarkable capacity for quickly evolving into new species to seize control of unexploited niches in the environment. And among these ecological opportunists, there are few better examples than the cichlids, a group of freshwater fishes that are one of the most varied group of back-boned animals on the planet.

Malawi cichlidsIn the words of Edward O. Wilson, the entire lineage seems “poised to expand.” The Great Lakes of Africa – Tanganyika, Malawi and Victoria – swarm with a multitude of different species; Lake Malawi alone houses over 500 that live nowhere else in the world.

All of these forms arose from a common ancestor in a remarkably short span of time. Now, a new study suggests that this explosive burst of diversity has been partly fuelled by rivalry between hostile males.

Michael Pauers of the Medical College of Wisconsin found that male cichlids have no time for other males that look like them and will bite, butt and threaten those who bear the same colour scheme. In doing so, they encourage diversity in the lake since mutant males with different tints are less likely to be set upon by territorial defenders.

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Cross-breeding restores sight to blind cavefish

Blogging on Peer-Reviewed ResearchIn the caves of Mexico lives a fish which proves that a million years of evolution can be undone with a bit of clever breeding.

Blind cavefishThe blind cavefish (Astyanax mexicanus) is a sightless version of a popular aquarium species, the Mexican tetra. They live in 29 deep caves scattered throughout Mexico, which their sighted ancestors colonised in the middle of the Pleistocene era. In this environment of perpetual darkness, the eyes of these forerunners were of little use and as generations passed, they disappeared entirely. They now navigate through the pitch-blackness by using their lateral lines to sense changes in water pressure.

But there is a deceptively simple way of restoring both the eyes and sight that evolution has taken, and Richard Borowsky from New York University’s Cave Biology Research Group has found it. You merely cross-bred fish from different caves.

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Evolutionary arms race turns ants into babysitters for Alcon blue butterflies

Blogging on Peer-Reviewed ResearchIn the meadows of Europe, colonies of industrious team-workers are being manipulated by a master slacker. The layabout in question is the Alcon blue butterfly (Maculinea alcon) a large and beautiful summer visitor and its victims are two species of red ants, Myrmica rubra and Myrmica ruginodis.

Myrmica rubra and an Alcon blue butterfly caterpillarThe Alcon blue is a ‘brood parasite’ – the insect world’s equivalent of the cuckoo. David Nash and European colleagues found that its caterpillars are coated in chemicals that smell very similar to those used by the two species it uses as hosts. To ants, these chemicals are badges of identity and so similar are the caterpillars that the ants adopt them and raise them as their own. The more exacting the caterpillar’s chemicals, the higher its chances of being adopted.

The alien larvae are bad news for the colony, for the ants fawn over them at the expense of their own young, which risk starvation. If a small nest takes in even a few caterpillars, it has more than a 50% chance of having no brood of its own. That puts pressure on the ants to fight back and Nash realised that the two species provide a marvellous case study for studying evolutionary arms races (which I’ve blogged about before here).

Theory predicts that if the parasites are common enough, they should be caught in an ongoing battle with their host, evolving to become more sophisticated mimics, while the ants evolve to become more discriminating carers. The two species make a particularly good model for this because their geographical ranges overlap in a fractured mosaic.

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Mud time capsules show evolutionary arms race between host and parasite

Blogging on Peer-Reviewed ResearchEvolution can sometimes be seen as a futile contest. Throughout the natural world, pairs of species are locked in an evolutionary arms race where both competitors must continuously evolve new adaptations just to avoid ceding ground. Any advantage is temporary as every adaptive move from a predator or parasite is quickly neutralised by a counter-move from its prey or host. Coerced onward by the indifferent force of natural selection, neither side can withdraw from the stalemate.

Mud time capsules show evolutionary arms race between host and parasiteThese patterns of evolution are known as Red Queen dynamics, after the character in Lewis Carroll’s Through the Looking Glass who said to Alice, “It takes all the running you can do, to keep in the same place.”

These arms races are predicted by evolutionary theory, not least as an explanation for sex. By shuffling genes from a mother and father, sex acts as a crucible for genetic diversity, providing a species with the raw material for adapting to its parasites and keep up with the arms race.

Watching the race

We can see the results of Red Queen dynamics in the bodies, genes and behaviours of the species around us but actually watching them at work is another matter altogether. You’d need to study interacting species over several generations and most biologists have neither the patience nor lifespan to do so.

But sometimes, players from generations past leave behind records of the moves they made. Ellen Decaestecker and colleagues from Leuven University found just such an archive in the mud of a Belgian lake.

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Bdelloid rotifers – 80 million years without sex

Sex is, on the whole, a good thing. I know it, you know it, and natural selection knows it. But try telling it to bdelloid rotifers. These small invertebrates have survived without sex for some 80 million years.

Bdelloid rotifers under the microscopeWhile many animals, from aphids to Komodo dragons, can reproduce asexually from time to time, it’s incredibly rare to find a group that have abandoned sex altogether. The bdelloid rotifers (pronounced with a silent b) are an exception.

They live in an all-female world and since their discovery, not a single male has ever been found. Genetic studies have confirmed that they are permanently asexual, and females reproduce by spawning clone daughters that are genetically identical to them.

The bdelloids pose a problem for evolutionary biologists, who have struggled to explain how they could make do without a strategy that serves the rest of the animal kingdom very well. Now, Natalia Pouchkina-Stantcheva, Alan Tunnacliffe and colleagues from the University of Cambridge have found out how they do it.

Sexual animals have two copies of each gene that have only minimal differences between them. But the asexual bdelloid lifestyle has uncoupled the fates of each copy in a gene pair, allowing them to evolve in new directions. They get two genes for the price of one.

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Genetic diversity gives honeybees an edge

Worker bees from more genetically diverse hives are more capable of dealing with day-to-day tasks.Social insects like ants, bees and wasps are some of the most successful animals on the planet. By acting as large super-organisms, they can achieve things that larger singular creatures cannot.

Their astounding selflessness is driven by an unusual way of handing down their genes, which means that females actually have more genes in common with their sisters than they do with their own daughters. And that makes them more likely to put the good of their colony sisters over their own reproductive legacy.

The more related the workers are to each other, the more willing they will be to co-operate. So you might expect colonies of social insects with fairly low genetic diversity to fare best. But that’s not the case, and Heather Matilla from Cornell University has found that exactly the opposite is true for bees.

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Butterflies evolve resistance to male-killing bacteria in record time

Six years ago, the males of a Samoan butterfly were being killed off by a bacteria and made up only 1% of the population. Now, the males have returned in a dramatic comeback and the sex ratio is equal again. In just ten generations, they evolved resistance to the parasite, a powerful example of natural selection in action.

Males of the blue moon butterfly has staged an amazing comeback.In our world, there is (roughly) one man for every woman. Despite various social differences, our gender ratio remains steadfastly equal, so much so that we tend to take it for granted. Elsewhere in the nature, things are not quite so balanced.

Take the blue moon butterfly (Hypolimnas bolina). In 2001, Emily Dyson and Greg Hurst were studying this stunningly beautiful insect on the Samoan islands of Savaii and Upolu when they noticed something strange – almost all the butterflies were females. In fact, the vastly outnumbered males only made up 1% of the population.

Male-killers

The cause of this female-dominated world was an infection, an inherited bacterium called Wolbachia. Wolbachia is a strong candidate for the planet’s most successful parasite for it infects a huge proportions of the world’s arthropods, themselves a highly successful group. And it does not like males.

Wolbachia has an easy route of infection – it can be passed to the next generation through the eggs of an infected female. But it can’t get into sperm, and for that reason, male insects are useless to it and it has a number of strategies for dealing with them.

Sometimes, it allows females to reproduce without male fertilisation. At other times, it forces males to undergo sex changes to become females. But in cases like the blue moon butterfly, it simply kills the males outright before they’ve even hatched from their eggs.

In 2001, Dyson and Hurst noted that the islands with the fewest males were the ones with the most prevalent Wolbachia infections.

The butterflies fight back

Female blue moon butterflies dominated Samoa until recently, thanks to Wolbachia infectionsBut by 2005, things had changed. Sylvain Charlat from University College London, along with Hurst and others, found that males were increasing in number all around Upolu Island. A year later, a formal survey confirmed the males’ amazing comeback.

On Upolu, they equalled in the females in number. Within just 10 generations, the male butterflies had gone from being outnumbered a hundred to one to an equal footing with the females. “To my knowledge, this is the fastest evolutionary change that has ever been observed,” said Charlat.

Charlat found the same story at a site on Savaii Island close to neighbouring Upolu. On the other side of the island, the males were still in the minority and many failed to hatch. But at 7% of the population, they were doing better than they had done in five years before.

All the butterflies were still infected with the same Wolbachia strain that had slaughtered their males just a few years back. And the bacteria themselves had not changed – when Charlat mated infected females with uninfected males from a different island, the parasite’s male-killing nature resurfaced within a few generations.

Evolution in action

Charlat believes that the Upolu butterflies had gained a resistance gene (or several) that allowed them to shrug off the male-killing bacteria. It either evolves the trait itself, or gained it from South-east Asian populations that had already become resistant.

Whatever the origin, the mutation spread like genetic wildfire across the Upolu and onto neighbouring Savaii. Most mutations carry small benefits and spread slowly. But by levelling a populations sex ratio, a mutation that resists Wolbachia clearly provides a huge advantage.

Male blue moon butterflies have evolved resistance to Wolbachia in record time.Surviving males who carry the gene(s) would have had their pick of females, since most of their competition lay dead in their eggs. And females, that picked up the mutation would have had twice the number of surviving young.

Arms race

Charlat’s work highlights just how powerful an influence parasites have in the course of evolution. Just how powerful parasites can be in the course of evolution. Events like this may be very commonplace, but at such speed, they may have happened before researchers could spot them.

The butterflies’ newfound resistance is also marvellous example of the Red Queen hypothesis, where parasites and hosts are caught in an evolutionary arms race. Each is forced to acquire new adaptations and counter-adaptations just to stay in the same place.

In this particular arms race, the butterflies have won the battle against Wolbachia. But the war isn’t over. The parasite now faces renewed pressures to find innovative ways of doing away with the dead-end males. How long will it be before it evolves a retaliatory strike?

More on evolution in action:
Natural selection does a handbrake turn – quick evolution at work
Of flowers and pollinators – a case study in punctuated evolution

More on parasites and evolutionary arms races:
Parasites can change the balance of entire communities
Viruses evolve to be more infectious in a well-connected population
Beetle and yeast vs. bee – how American bees are losing the evolutionary arms race

More on animal sex and reproduction:
Virgin birth by Komodo dragons
When the heat is on, male dragons become females
Chimerism, or How a marmoset’s sperm is really his brother’s
Aphids get superpowers through sex

 

Reference: Charlat, Hornett, Fullard, Davies, Roderick, Wedell & Hurst. 2007. Extraordinary flux in sex ratio. Science 317: 214.

 

 

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Photos by Sylvain Charlat and Comacontrol

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.

The gray wolf - smaller than the Beringian variety, and with weaker jawsThis 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.

The eastern Beringian wolf was a formidable hunter that could also turn to scavenging - just like modern hyenas.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.

The extinct super-wolf would have been able to hunt prey even larger than this bison.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

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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

Of flowers and pollinators – a case study in punctuated evolution

Darwin predicted that flowers and pollinators are engaged in an evolutionary arms race. But that’s not always the case. In a group called columbines, evolution happened in a stop-start ‘punctuated’ way, as the flowers encountered new pollinators with increasingly long tongues.

Charles Darwin was a visionary in more ways than one. In 1862, Darwin was studying a Malagasy orchid called Angraecum sesquipedale, whose nectar stores lie inaccessibly at the bottom of a 30cm long spur (tube). Darwin predicted that the flower was pollinated by a moth with tongue long enough to raid the spur.

Darwin’s moth, perfectly adapted to drink from a long-spurred orchid.Few people believed him, but in 1903, zoologists discovered Darwin’s predicted moth, Xanthopan morgani praedicta, and it did indeed have a very long tongue. Darwin’s accurately predicted the extraordinary but matching lengths of moth tongue and orchid spur, but his explanation for them is another story.

The arms race model

He suggested that the two species were locked in an ‘evolutionary arms race’. Orchids and pollinators gradually co-evolved over time, lengthening both tongues and spurs in response to each other.

Orchids with the longest spurs have an advantage. Their nectar stores are only just within reach of pollinators, so they are tempting but don’t sacrifice too much valuable nectar. For pollinators, the advantage belongs to those with the longest tongues because they have access to the most food.

The arms race model has become widespread and popular since Darwin’s time. It helps to explain relationships between predators and prey, parasites and hosts and even males and females. But its original function – to explain the relationship between flowers and pollinators – has just been called into question.

Columbines

Justen Whittall and Scott Hodges from the University of California, Santa Barbara, tested the arms race theory by looking at another long-spurred flowering plants – the columbines (Aquilegia sp). In these flowers, every petal carries its own elongated nectar spur and the advent of these spurs coincided with the recent and rapid diversification of this group.

Columbines of North America have a great range of spur sizesWhittall and Hodges charted the evolutionary relationships between the 25 North American columbine species, whose spurs range form barely a centimetre in length, to just over twelve. They found that this great variety of lengths was driven by changes in pollinators, rather than gradual races against a single one.

The flowers with the shortest spurs were pollinated by short-tongued bumblebees. Hummingbirds, whose tongues are longer, pollinate columbines with longer spurs, while hawk-moths, with the longest tongues of all, carry the pollen of the longest-spurred flowers.

There is no overlap between these three groups and once a lineage switches pollinator it doesn’t go back. Over the course of their evolution, the columbine lineages went from bumblebees to hummingbirds, and then to hawkmoths, lengthening their spurs with every jump.

The ‘pollinator shift’ model

Based on these observations, Whittall and Hodges put forward an alternative to Darwin’s arms race model. They imagined a columbine ancestor that was well adapted to the tongue length of a specific pollinator (say, a bumblebee).

In part of its range, the flower started to be visited by a second pollinator (say, a hummingbird) with a much longer tongue. In this area, the plant rapidly evolved a longer spur in response to its new partner and over time, this led to two species with different spur lengths and different pollinators.

In this model, the columbines’ spurs evolved in a ‘punctuated’ stop-start way, very different to Darwin’s model of gradual change. Each pollinator shift triggered a large evolutionary rush, as the species lengthened their spurs in response to the longer tongues of their new partners. In between these shifts, the pace of evolution slowed down considerably.

A columbine flower - its long spurs are driven by evolutionary shifts between pollinatorsBut Darwin’s arms race idea isn’t out for the count yet. Whittall points out that columbines that are pollinated by hawkmoths have a great variety of spur lengths themselves that were most likely the result of an arms race. And the moths themselves evolved long before the columbines did, so the variations in their tongue lengths must have evolved in relationships with other plants.

An adaptive valley

The stop-start model also explains a difference between columbines around the world. Those in Europe and Asia have a much smaller range of spur lengths than their North American cousins, and none of them are pollinated by hawkmoths. Whittall and Hodges have an answer for this too – it’s because Eurasia has no hummingbirds.

Imagine if flowers tried to make the evolutionary leap from bumblebee to hawkmoth without the intermediate stepping stone of hummingbirds. At the intermediate spur length, the flower would have excluded its old pollinator, whose tongues would now be too short to reach any nectar. But it would have no advantage over its new pollinator, whose amply long tongues could drink the flowers dry.

Between bumblebee and hawkmoth lies an ‘adaptive valley’, where intermediate-length flowers have no advantage and are ignored by natural selection. In Eurasia, there is not enough impetus for a species to cross it. But North America, the hummingbirds act as a stepping stone that allowed the columbines to ford this gap and evolve even more extreme flowers.

Reference: Whittall & Hodges. 2007. Pollinator shifts drive increasingly long nectar spurs in columbine flowers. Nature 447: 706-709.

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Images: Moth from MSN Encarta, columbine montage from Justen Whittall’s website.

Related stories about evolution:
Natural selection does a handbrake turn – quick evolution at work
Salamander robot walks, swims and sheds light on evolutionary step from sea to land
Human cone cell lets mice see in new colours
Living optic fibres bypass the retina’s back-to-front structure
Viruses evolve to be more infectious in a well-connected population
The evolution of animal personalities – they’re a fact of life

Simple sponges provide clues to origin of nervous system

The possible origins of the nervous system have been found in the simple sponge, an animal with no nervous system of its own. Sponges carry the genetic components of synapses, which may have been co-opted by evolution as a starting point for proper nerve cells

Sponges are the most primitive of all animals. They are immobile, and live by filtering detritus from the water. They have no brains or, for that matter, any organs, tissues or nervous system of any sort. If you were looking for the evolutionary origins of animal intelligence, you couldn’t really pick a less likely subject to study.

Over time, evolution co-opted the early PSD of the sponge and used it to craft true nervous systems.So it was with great surprise that Onur Sakarya from the University of California, Santa Barbara found that sponges carry the beginnings of a nervous system.

With no neurons to speak of, these animals still have the genetic components of synapses, one of the most crucial parts of the nervous system. And their versions share startling similarities with those of humans.

Synapses (and proto-synapses)

Synapses are junctions between two nerve cells that are allow the cells to pass on signals to each other. Signals are carried by molecules that cross the synaptic gap called neurotransmitters. When they reach the receiving cell, they come across an elaborate tangle of proteins called the post-synaptic density (PSD; labelled in red below). The PSD processes the neurotransmitters, among many other important roles, and allows the receiving cell to respond appropriately to the nervous signal

Sakarya searched for equivalents of the human PSD proteins in the genomes of other animals. For a start, he found an almost complete set in the starlet sea anemone (Nematostella vectensis). The anemone (like its cousins, the jellyfish) is one of the Cnidarians, a group of animals that have the most rudimentary of nervous systems. Finding PSD genes in them is surprising but reasonable.

The synapse relays signals from one cell to another.But Sakarya was really surprised when he found the vast majority of the PSD assemblage in the sponge Amphimedon queenslandica, an animal that doesn’t even have a nervous system! The sponge’s PSD proteins bore remarkable resemblances to those of humans and other animals, and were built of similar arrangements of domains.

One in particular, the PDZ domain, allows PSD proteins to recognise one another and assemble correctly. When Sakarya compared the structures of the human and sponge PDZ domains, he found that at the atomic level, the parts they used to interact with other proteins were almost 90% identical. So not only does the sponge have the full set of PSD parts, it can assemble them into a fully-functioning whole.

Pre-adapted

So what is the PSD, part of the nervous system, doing in an animal without one? Sakarya believes that the PSD is an example of exaptation, a process where evolution co-opts an existing structure for another purpose. Bird feathers are a good example of this – they evolved in small dinosaurs to help them regulate their body temperature, and were only later used for flight.

Exaptation can explain how complex, integrated structures like the nervous system can evolve. Rather than building the whole thing from scratch, evolution took ‘off-the-shelf’ components, like the PSD, and put them together in exciting new ways.

Sponges are the simplest of animals but even they have genetic components of synapsesIn the same way, the PSD of sponges is switched on in a type of cell called the ‘flask cell’. Flask cells are only found in sponge larvae, which, unlike the adults, are free-swimming. These cells could help the larvae to sense their environment, and could well have been a starting point for the evolution of neurons.

Sakarya cautions that there could be another explanation. Sponges could be degenerate relics of a more advanced branch of animals, that stripped away their complexity in favour of life in the (very) slow lane. In this scenario, the flask cells are evolutionary remnants of neurons proper.

Nonetheless, under both scenarios, these findings strongly suggest that the common ancestor of all living animals already has an early working version of the PSD. This practically pre-adapted it for the evolution of nervous systems. With minimal additional evolutionary steps, this early scaffold could have been transformed into the functional synapses that drive our thoughts today. The ancestor was pre-adapted to a future with neurons.

It’s worth noting that this discovery was only made possible because the genome of Amphimedon has been fully sequenced. In an age where genome sequencing could start to be taken for granted, this drives home the importance of sequencing a wide variety of living things that represent crucial junctures in evolution.

Reference: Sakarya, Armstrong, Adamska, Adamski, Wang, Tidor, Degnan, Oakley & Kosik. 2007. A post-synaptic scaffold at the origin of the animal kingdom. PLOS One 6, e506: 1-7.

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Related stories about evolution:
Of flowers and pollinators – a case study in punctuated evolution
Natural selection does a handbrake turn – quick evolution at work
Salamander robot walks, swims and sheds light on evolutionary step from sea to land
Human cone cell lets mice see in new colours
Living optic fibres bypass the retina’s back-to-front structure

Related stories about nervous systems:
Maternal hormone shuts down babies’ brain cells during birth
No new brain cells for you – settling the neurogenesis debate
Bats create spatial memories without making new brain cells

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