May 30 (UPI) -- Determining when life emerged is a difficult question, but it's one many scientists remain interested in.
New research questions the accuracy of a recent model that suggests the first animals emerged 1.2 billion years ago, several hundred million years earlier than the oldest fossil.
The oldest known fossil is a little more than 500 million years old. But every few years, an older fossil is discovered. In recent decades, scientists employed an alternative, fossil-free approach to dating the origin of animal life.
In the 1960s, Nobel winner Linus Pauling pioneered a genetic measurement method known as the molecular clock. The logic of the molecular clock relied on the assumption that genetic mutations accumulate in the genomes of animal lineages at a fixed rate. Early uses of the method estimated that the earliest animal ancestor existed roughly 1.5 billion years ago.
The prediction sparked intense debate among evolutionary sciences. Many researchers challenged the assumption that mutations accumulate at a fixed rate.
Over the last decade, molecular clock calculations have relaxed the rate of mutation accumulation and the divergence of animal lineages. The relaxed clock models suggest the first animals emerged no more than 850 million years ago.
However, a newer molecular clock model, dubbed RelTime, dates the origin of animal life to 1.2 billion years ago.
Researchers in Europe decided to take a closer look at the math behind the RelTime clock. They published a critique of the model in the journal Genome Biology and Evolution.
"What caught our attention was that results obtained using RelTime were in strong disagreement with a diversity of different studies, from different research groups and that used different software and data, all of which broadly agreed that animals are unlikely to be older than approximately 850 million years," Philip Donoghue, an earth scientist at the University of Bristol, said in a news release.
Most molecular clocks rely on Bayesian logic to relax the clock. The RelTime clock uses alternative statistical analysis methods.
"Estimating divergence times is difficult and different relaxed molecular clock methods use different approaches to do so," said Jesus Lozano-Fernandez, a scientist from the University of Bristol. "However, we discovered that the RelTime algorithm failed to relax the clock along the deepest branches of the animal tree of life."
Researchers argue the RelTime model suffers from the same inaccuracies as the earliest molecular clocks.
"This clearly indicates that older ideas suggesting that animals might be twice or three times as old as the oldest animal fossil are erroneous and only emerge when changes in mutation rate are incorrectly estimated," said Bristol professor Davide Pisani.