Where Fossils Fear To Tread
Scientists Follow Genes To An Ancient Ancestor

Some 600 or 700 million years ago, before animal life made a sudden evolutionary shift and diverged into nearly all the major animal divisions we know from fossils, primitive animals were inventing the genes that would make it all possible.

No one knows what it looked like. There is virtually no clear fossil evidence. But now scientists believe they have found a way – using genes preserved in and common to modern animals – to look past the fossil record to our most distant common ancestors.

Writing this week (Aug. 14) in the journal Nature, and following up on a series of recent papers, molecular biologist Sean Carroll of the Howard Hughes Medical Institute at UW–Madison, Neil Shubin from the University of Pennsylvania, and Cliff Tabin of Harvard Medical School, sketch out a radical new way of looking back in time.

“The fossil record prior to the Cambrian is so scant nobody knows the origin of animal life,” said Carroll. But now “we’re drawing a picture of something no one has ever seen.”

Carroll is one of a growing number of scientists now using the techniques of modern molecular biology to look into the murky waters of distant evolutionary biology. They are looking so far back that there are virtually no fossils or other physical clues to what the Earth’s earliest animals were like.

But Carroll and others are now finding powerful evidence that an ancient common ancestor – a worm-like animal from which most of world’s animals subsequently derived – invented a set of body-building genetic machinery so successful and malleable that it has survived to this day.

“This is stunning,” said Carroll. “Nobody thought that this animal was so sophisticated. We’re talking about the common ancestor of all the most successful animals on Earth.”

What is so striking, according to Carroll, is that the genes used to grow appendages – legs, arms, claws, fins and antennas – were operational at least 600 million years ago, and that the genetic machinery is very similar in all animals past and present. What makes animals different, what differentiates a crab from a mouse or a fruit fly from an eagle, is simply how those genes are expressed, he said.

Until the advent of genetic techniques and recent work that has shown that animals, as embryos, share the same genetic machinery that governs body architecture, the only recourse for understanding how animals evolved different kinds of appendages was in the realm of comparative anatomy.

And appendages, said Carroll, have been used as classic examples of independent evolution. But now Carroll and his colleagues argue that the problem of developing limbs, be they claws or wings, was solved just once a very long time ago, and that the genetic mechanism is still at work.

“Everybody thought the wheel was invented again and again and again,” Carroll said, “but there was a single solution and everything is a modification of that.”

That argument is supported, said Carroll, by the discovery of the same appendage-making genes in six broad divisions of the animal kingdom, including vertebrates, insects and fish. The discovery, made in Carroll’s lab, was reported last May in the Proceedings of the National Academy of Sciences.

“We found the same mechanism in all of these divisions of the animal kingdom. The architecture can vary tremendously, but the genetic instructions are the same and have been preserved for a very long period of time,” he said.

The idea that a common set of genes is responsible for building appendages not only simplifies evolutionary history, but helps explain the great burst of evolutionary activity known as the Cambrian explosion. This “evolutionary big bang” took place in the world’s oceans more than 500 million years ago when new animals appeared at breakneck speed.

“The reality is that animals with appendages took off and dominated the Earth,” at that time Carroll said. “It was like an arms race” with animals that could swim faster, grab tighter and fight with greater effect dominating the ocean environment and conquering new ones like the land. During the Cambrian, animals got bigger and more diverse, but those changes did not require new genes.

The techniques being pioneered by Carroll and others are opening a new window to the past, said Carroll: “It’s doing paleontology without fossils.”

CONTACT: Sean Carroll, 608-262-6191, sbcarrol@facstaff.wisc.edu