Evolution’s Clock Ticked Faster at the Dawn of Modern Animals

 

15-09-2013

15

Michael Lee

Explosive evidence. New research combines data from living arthropods like this centipede with the fossil record of its 515-million-year-old ancestor, the trilobite, to measure the speed of evolution’s “big bang.”

Five hundred thirty million years ago, the number and diversity of life forms on Earth mushroomed. This so-called Cambrian explosion kept Charles Darwin, the father of evolution, awake at night, as he worried that his theory of natural selection couldn’t explain the sudden proliferation of species. Now, researchers have combined evidence from the fossil record with clues in the genes of living species to estimate the speed of that evolutionary explosion. Their finding—that the rate of change was high, but still plausible—may put Darwin’s fears to rest.

The dawn of the Cambrian period divides two very different Earths. In one, primitive, mostly single-celled creatures “sat on the mud and did very little,” says evolutionary biologist Matthew Wills of the University of Bath in the United Kingdom. In the other, life forms as diverse as our modern fauna roamed the planet. The abrupt appearance of these creatures in the fossil record “gave Darwin a headache,” Wills says, and critics of evolution have argued that the tree of life couldn't possibly produce so many branches and bear such a variety of fruit so quickly.

Some scientists explained away this dilemma by claiming that the fossil record is deceptive. Perhaps, they speculated, the first representatives of modern animal groups appeared long before the Cambrian period, but had tiny, soft bodies what were not easily preserved as fossils. But based on fossil evidence, most paleontologists believe the “fuse” on the explosion must have been short, with new life forms proliferating only a few tens of millions of years before the Cambrian period. Just how quickly would species have to evolve to squeeze in all these new developments?  “No one has actually tried to quantify just how fast the rates were,” says Michael Lee, an evolutionary biologist at the University of Adelaide in  Australia and the South Australian Museum, who led the new research. “They just literally took Darwin’s word that they must have been pretty fast.”
So Lee and colleagues estimated that speed by studying the evolution of arthropods—Earth’s most diverse phylum, which includes insects, crustaceans, and arachnids. They looked at how changes evolved in both the genetic code and the anatomy of arthropods, comparing 62 different genes and 395 physical traits. For any two branches of the arthropod family tree—centipedes and millipedes, for example—they picked out important physical differences and variations in genetic sequence in modern specimens. Then, using evidence from the fossil record about how quickly the two branches diverged, the group calculated roughly how fast genetic and anatomical differences must have emerged for each lineage over time.

They found that when some early branches of the arthropod family tree were splitting off, creatures were evolving new traits about four times faster than they did in the following 500 million years. The creatures' genetic codes were changing by about .117% every million years—approximately 5.5 times faster than modern estimates, the group reports online today in Current Biology. Lee calls this pace “fast, but not too fast” to reconcile with Darwin’s theory.

This combined model for genes and anatomy represents “quite a stride forward,” Wills says. The results not only show that the evolutionary clock ticked much faster around the time of the Cambrian, but also hint at what may have sped it up. The fact that genes and anatomy evolved at roughly the same rate suggest that pressures to adapt and survive in a world of new, complex predators drove both, the authors speculate. Innovations such as exoskeletons, vision, and jaws created new niches and evolution sped up to fill them. Wills agrees that the new research makes this explanation for the Cambrian explosion “look a lot more probable now.”

Others caution that such analysis is in its infancy. “It’s an excellent first step,” says Douglas Erwin, a paleontologist at the Smithsonian Institution in Washington, D.C., but the exact rates of evolution in the study might not be reliable. He points out that while the study uses fossil data to determine when a given arthropod branch emerged, it doesn’t include the known characteristics of these extinct ancestors in its comparisons of physical traits, which involve only living creatures.

Some of the assumptions the authors make in estimating these emergence dates are also problematic, says Philip Donoghue, a paleobiologist at the University of Bristol in the United Kingdom. But he believes future iterations of this approach—incorporating fossil traits into the analysis—will yield a powerful new tool: “All the cool kids will be doing it soon.”