Biologists have identified an ancient common ancestor which invented a set of flexible body-building genetic machinery that survives to this day. Roger Highfield reports
A WORM-LIKE animal that wriggled on Earth 700 million years ago played a crucial role in shaping the body plans of living things today, from the legs of a supermodel to the wings of a bee.
The creature, dubbed Urbilateria, invented a set of body-building genetic machinery so malleable and flexible that it is responsible for the diversity of life on the planet, be it wriggling, crawling, walking or flying.
Traditionally, scientists used a straightforward approach to understanding body plan: they compared creatures by analysing anatomy, counting fingers, toes, and body segments, then tried to deduce how shape and form evolved.
Now, however, they are scouring the genetic code of living things for genes that determine body shape. From these they can make deductions about the first genetic machinery to alter the architecture of multi-cellular creatures.
Writing in the current issue of Nature, Dr Sean Carroll, of the Howard Hughes Medical Institute at the University of Wisconsin-Madison, Dr Neil Shubin, from the University of Pennsylvania, and Dr Cliff Tabin, of Harvard Medical School, describe the latest insights to emerge from the field of evolutionary developmental biology, or "evo-devo".
Genetics allows them to investigate body plans from eras so ancient there are virtually no physical clues to what these earliest animals were like. "It's doing palaeontology without fossils," said Dr Carroll.
They believe that Urbilateria, an ancient common ancestor from which most of the world's animals derived, invented a set of flexible body-building genetic machinery that has survived to this day. "This is stunning," said Dr Carroll. "We're talking about the common ancestor of all the most successful animals on Earth."
Intriguingly, scientists have no fossils of Urbilateria, so don't know what it looked like. But molecular biology suggests that genes used to grow appendages - legs, arms, claws, fins and antennae - were operational in this worm at least 600 million years ago.
Dr Carroll and his colleagues argue that the problem of developing limbs was solved at that time - and that the genetic mechanism is still at work. What makes animals different, what differentiates a crab from a mouse, is how those genes are put to work.
"Everybody thought the wheel was invented again and again and again," Dr Carroll said, "but there was a single solution and everything is a modification of that."
That argument is supported, said Dr Carroll, by the discovery of the same appendage-making genes in six broad divisions of the animal kingdom, including vertebrates, insects and fish.
The idea that a common set of genes is responsible for building appendages simplifies evolutionary history and helps to explain the burst of evolutionary activity known as the Cambrian explosion, an "evolutionary big bang" when new animals emerged at breakneck speed in the world's oceans more than 500 million years ago.
"Animals with appendages took off and dominated the Earth," Dr Carroll said. "It was like an arms race." Animals that could swim faster, grab tighter and fight harder dominated the ocean environment and conquered new ones, like land.
The body-building instructions of Urbilateria have been modified and elaborated to provide what today determines the body plan, such as the "homeotic" genes that divide the head-to-tail axis into distinct zones.
Dr Nipam Patel, of the University of Chicago, and Dr Michalis Averof, of the European Molecular Biology Laboratory, Heidelberg, showed that changes in the pattern of activity of these genes in crustaceans are linked to the relatively sudden evolutionary development of feeding limbs called maxillipeds (literally jaw-feet), where swimming or walking legs once were.
They focused on two genes, Ubx and abdA, that define what kinds of appendage appear on crustaceans. To show in which segments of embryos the genes were at work, they used an antibody.
At the outset, all the limb buds of the head, thorax and abdomen look the same. The kind of appendage that grows, whether for swimming or walking, depends on these homeotic genes, which tell the limb buds where they are on the body.
The researchers found variation in the pattern of segments in which Ubx-abdA is turned on, and that pattern corresponded with anatomical changes that traced ancestral relationships.
In primitive crustaceans, Ubx-abdA is turned on in the first thoracic segment and is "on" from there back to the tail. This results in a marked difference between the appendages of the head and the thorax: the former has tiny appendages for pushing food into the mouth, while the latter has long, feathery appendages for swimming.
In advanced crustaceans, such as shrimps and lobsters, the first segment in which Ubx-abdA is turned on lies farther back along the body, so that the first thoracic segments have appendages akin to feeding appendages on the head, to scrape and carry food.
The finding demonstrates how changes in the control of homeotic genes underlie an evolutionary trend leading to novel body structures. With complementary work on vertebrates by Cliff Tabin, and on fruit flies and butterflies by Sean Carroll, scientists have passed an evo-devo landmark.
