CALL it CSI: Precambrian. About 700 million years ago, one of the most significant – and most mysterious – events in the history of life on Earth occurred. Suddenly, there was more to life than just single-celled microbes. Within a few tens of millions of years, an extraordinary array of large animals appeared, armed with jaws and claws and eyes and brains.
Yet we still know surprisingly little about the origin of multicellular animals. “The different branches of the animal tree evolved very rapidly in a short period, a long time ago,” says Nicole King, an evolutionary biologist at the University of California, Berkeley.
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The very first animals left few fossil traces. What they did leave were lots of descendants. It is to these descendants that evolutionary detectives have to turn to reconstruct the events of those early years. By comparing the genes of living organisms and painstakingly working out their family trees, they are slowly building up circumstantial evidence and piecing together a sketch of that first animal, our great-to-the-nth-grandmother. And not just its appearance – the detectives are also coming up with a motive, a reason why animals evolved when they did.
See how it happened: Six steps from single cells to complex animals
“Our great-to-the-nth-grandmother was a hungry sponge larva. Put that on your family tree”
The closest living relatives of multicellular animals, or metazoans, have long been considered to be an obscure group of single-celled creatures called choanoflagellates. These little microbes filter food from water using a tail, or flagellum, set in the middle of a crown-like collar. The feeding cells of sponges bear a similar collar – a hint that the earliest metazoan might have been sponge-like. More recently, DNA sequence comparisons have confirmed this close kinship.
However, sponges seem unlikely ancestors. They are little more than a loose assemblage of cells that lack the true tissues and organs found in higher animals. Plus their structure – a porous mass of interconnected water channels – is nothing like that of any other animal. As a result, most zoologists tend to put sponges on a side branch of the animal tree, an abortive experiment that led nowhere. That’s what makes Kevin Peterson’s so thought-provoking.
A bath sponge
Peterson, a molecular palaeobiologist at Dartmouth College in Hanover, New Hampshire, and his colleagues compared the sequences of seven genes from 42 species of animal, and then used sophisticated computer programs to assemble the species into the most likely evolutionary tree. Sure enough, sponges branch out near the base – but Peterson’s tree differs from most others in a subtle but hugely significant way. Instead of putting sponges on a single side branch, Peterson’s tree has sponges on both sides of the trunk.
In other words, the ancestor of all complex animals not only resembled a sponge, it actually was a sponge. “If you had a time machine and brought back the last common ancestor of all living animals, and you gave it to an invertebrate zoology class, they’d call it a bath sponge,” says Peterson.
Peterson’s conclusion is highly controversial. Other research teams, using similar analyses but an alternative selection of species and different DNA sequences, have fingered other animal groups as most resembling the ancestor.
The mysterious placozoan
For example, a study based on DNA sequences and morphological characteristics of 24 metazoans suggests that the ancestral metazoan most likely resembled not a sponge but an obscure modern-day animal called a placozoan ().
Placozoans are little more than sheets of cells a few millimetres long, with no gut, nerves or muscles. Despite this, they can . Their simplicity makes them a likely model for the common ancestor – except that there might be more to placozoans than we know. Their sex lives are a mystery, for instance, even though DNA analyses show they must have sex.
Another study, of 150 genes in 77 species, puts yet another animal group, the jellyfish-like ctenophores, at the base of the metazoa (). Others scoff at this possibility, since ctenophores are relatively sophisticated predators, which seem unlikely to have evolved before there were other large organisms to prey on.
Early days
Just last month, a new animal tree based on 128 genes from a wide range of animals put sponges as a separate group, distinct from the lineage leading to higher animals (). If this is right, the ancestral metazoan might not have been spongelike in form. Peterson, however, says his latest analysis – as yet unpublished – shows this tree is incorrect.
Perhaps all one can conclude from these attempts to sort out the base of the animal tree is that it is still early days. “So far, the data sets we have do not show robustness. When you add new species, you find different things,” says evolution biologist Antonis Rokas of Vanderbilt University in Nashville, Tennessee. “That makes me think it is more wobbly than these authors would have us believe.”
On balance, though, most experts think Peterson is probably on the right track. “I think the weight of evidence favours the notion that sponges evolved first,” says King. “These other observations are interesting, and we can’t rule out the possibility that they might gain more support, but right now the majority of the field is comfortable with the notion that sponges are the earliest-branching.”
Bizarre anatomy
So how does that square with the bizarre anatomy of sponges, which would seem to rule them out as the ancestor of higher animals? It is difficult to envision a stepwise progression from adult sponges to more complex animals – but it probably did not happen that way, says Claus Nielsen, an evolutionary morphologist at the University of Copenhagen in Denmark.
Sponges produce free-swimming larval forms not unlike some conceptions of the ancestral animal. These larvae typically live off stored nutrients and settle on the sea floor after a few days. The larvae of one early sponge, however, might have evolved ways of feeding during the larval stage and could thus have remained free-swimming for far longer than normal. Eventually these “larvae” might have begun breeding too, abandoning the “adult” bottom-dwelling stage altogether.
In this scenario, most of the anatomical problems disappear, Nielsen says. He has set out in detail (see “Six steps from single cells to complex animals“) how sponge larvae may have evolved into higher animals ().
No fossil record
All the intermediate organisms in the progression make sense as living, feeding animals – something that is not necessarily true of other, competing scenarios, Nielsen says. “The problem with older theories that started with a ball of cells is that nobody has speculated on how this organism could feed.”
A scenario like Nielsen’s would help make sense of the sudden burst of animal forms that appears in the fossil record, says Peterson. A world filled solely with soft-bodied sponges filtering bacteria and organic debris from the seawater would leave few fossils and show little morphological change. “Sponges would be palaeontologically invisible,” he says. What’s more, with nothing but sponges around, there would have been little selection pressure to drive further evolution. “There are no predators in the world’s biota. There are no arms races. Effectively, there’s no macroevolution.”
Then, about 700 million years ago, the Earth’s oceans froze over, or nearly so, at least once. During this “Snowball Earth” phase, there would have been fewer and fewer suitable spots for sponge larvae to settle on, says Peterson. Larvae that could drift with the currents for longer would have had a better chance of finding a suitable spot – and any larvae that could take in food while they drifted would thus have had a huge advantage.
Arms race
Eventually, Peterson speculates, some evolved a rudimentary gut. “Most of the genes are there. The ability to digest is there. You just need to get that opening,” he says.
With the help of crude guts, some of these early animals may have started to feed on others. This emergence of multicellular animals that preyed on other multicellular animals – however primitive – would have changed everything. Predators capable of chasing and overpowering their prey would have a huge advantage. Potential prey would have to detect and evade the predators to avoid becoming another meal.
The result is an arms race, says Peterson. “You just get better and better with muscles and nervous systems and sense organs.” That would have set the stage for the explosive diversification of animal forms that we see in the fossil record.
“You’ve got these two remarkable singularities – Snowball Earth and the origin of complex animals – at the same time. Is it just a coincidence? I don’t think so,” says Peterson. If he’s right, and if Nielsen’s scenario for animal evolution is accurate, then that shadowy being in the oh-so-distant past, our great-to-the-nth-grandmother, was a hungry sponge larva. Put that on your family tree.
Six steps from single cells to complex animals
Claus Nielsen of the University of Copenhagen, Denmark, has proposed that multicellular animals evolved from single-celled organisms in six major steps. Here is a simplified version of his proposed steps. The details of early animal evolution are still hotly debated, however, and his scenario is just one of many competing ones.
See how it may have happened:
