When Did The First Animals Evolve? New Study Debunks 1.2-Billion Year Contention

Life on Earth has existed for about 4.28 billion years, meaning it originated less than 350 million years after our planet formed, which is to say life has existed on Earth for over 90 percent of the planet’s existence. But the life-forms from that prehistoric time are markedly different from the flora and fauna we are used to seeing.

In fact, life continued as microorganisms and single-cell creatures for at least three billion years after that. And it has long been a subject of debate to establish when it was that animals finally evolved from their fewer-celled ancestors. Based on the dating of fossils, it was believed till the 1960s that the first animals appeared about half a billion years ago or thereabouts.

But fossils are rare, not very easy to interpret, and are constantly being discovered, sometimes older than any previous find. A new method, proposed in 1962 by Nobel Prize winner Linus Pauling and his colleague Émile Zuckerkandl, was based on the fact that the evolutionary change in a given protein moved approximately along a linear scale, with respect to time.

Read: Fossils Of Ancient See Animals Tell Us About Evolution

The early “molecular clock” studies, based on genetic information, assumed the mutation of proteins at a fixed rate across all species and calculated that the oldest animals lived about 1.5 billion years ago, or three times older than the oldest known animal fossils. The very large discrepancy led to the creation of more realistic molecular clock methods, called “relaxed” clock methods, and almost 50 years later, they predicted the oldest animals to have lived about 850 million years ago.

A recently developed relaxed molecular clock method, called RelTime, was used by researchers from universities of Oakland in Michigan and Temple in Philadelphia, who concluded the birth of first animals to have taken place about 1.2 billion years ago. While not quite three times the widely accepted figure, the studies still raised plenty of eyebrows and set some other researchers investigating the RelTime method.

“Estimating divergence times is difficult and different relaxed molecular clock methods use different approaches to doing so. However, we discovered that the RelTime algorithm failed to relax the clock along the deepest branches of the animal tree of life," Jesus Lozano-Fernandez from the University of Bristol, who was the lead author of a new paper that rejects the RelTime method, said in a statement Tuesday.

In essence, the problem with the RelTime method was that it was not sophisticated enough. Mario dos Reis, a study co-author from the Queen Mary University of London, explained: “Generally scientists use Bayesian methods to relax the clock. These methods use explicit probability models to account for the uncertainty in the fossil record and in the mutation rate. Bayesian methods borrow tools from financial mathematics to model variation in mutation rate in a way that is similar to that used to model the stochastic variation in stock prices with time. By applying these sophisticated mathematical tools, Bayesian methods relax the clock and estimate divergence times. However, RelTime is not a Bayesian method.”

So the recently published study, appearing in the journal Genome Biology and Evolution under the title “RelTime Rates Collapse to a Strict Clock When Estimating the Timeline of Animal Diversification,” sets the clock for the appearance of the first animals once again at about 850 million years ago.