ASU Research E-Magazine: Times of Life

Your family tree may be older than you think. Sudhir Kumar says to figure from half a billion to 1 billion years older, depending on how far you want to trace your lineage.

Kumar, an assistant professor of biology at Arizona State University, is also a member of NASA’s new astrobiology team. His research findings indicate that the ancestors of today’s insects and vertebrates diverged from each other 993 million years ago, more than 400 million years earlier than predicted by the fossil record. The major forms of multicellular life—animals, plants, fungi—apparently diverged another half billion years before that, at 1.6 billion years ago.

Kumar reported his findings in January 1999 in the Proceedings of the Royal Society of London. He co-authored the paper with Penn State University scientists Y.C. Wang and S. Blair Hedges. The scientists used “molecular clocks” to estimate the divergence points of chordates (the group containing mammals, fish, amphibians, reptiles, and birds), arthropods (insects and other animals with hard shells or exoskeletons), and nematodes (parasitic worms such as hookworms and pinworms).

By comparing the genes of species alive today, molecular clocks allow researchers to estimate how long ago major groups of animals, plants, and fungi split from one another.

Scientists know that mutations in the genome of a species accumulate over time, presumably at a relatively steady rate. As a result, by doing cross-species comparisons, researchers can date individual speciation events and reconstruct an evolutionary tree of life with relative accuracy. The more genes the researcher uses, the less statistical error is possible in time estimates.

Kumar and his Penn State colleagues used 50 genes from a variety of species to lessen their chances of statistical error. All of the team’s data came from large, public domain databases that have been constructed on the Internet.

Complex animals do not appear in the fossil record until the Cambrian period (about 530 million years ago), at which point many varieties suddenly appear in large numbers. These prehistoric organisms seem to appear virtually from nowhere. For decades, paleontologists and evolutionary biologists have assumed that the abundant and diverse Cambrian fossils represent an unparalleled boom in the creation of new species. They refer to this period in Earth’s evolutionary history as the “Cambrian explosion.”

New genetic evidence from studies like Kumar’s is bringing such Cambrian-dominated thinking into question.

“Our data pushes back the date of divergence more than 400 million years prior to what was previously believed,” Kumar explains. “This means that the Cambrian species may not have radiated so quickly. They might have diverged much more gradually.”

But, if complex life forms appeared on Earth hundreds of millions of years before the Cambrian period, why have so few pre-Cambrian fossils been found?

The fossil record is made up mostly of hard animal parts. Animals that lived before the Cambrian period may have been small and soft-bodied. Such organisms would probably not have been preserved as fossils, but may have had a long prior history nonetheless.

Kumar says that the molecular evidence does not contradict the fossil record, but complements it. At many points, the fossil and genetic evidence appears to converge.

“Molecular clocks will never replace the fossil record as a method for dating speciation events,” Kumar adds. “But genetic evidence will help us to fill in gaps in the fossil record and get answers in instances where there is little or no fossil evidence.” —Matthew Shindell