Winding the molecular clock back reveals pivotal human ancestor walked with dinosaurs

Molecular clock analysis suggests that placental mammals first emerged during the “Age of Dinosaurs,” surviving the asteroid impact which wiped out the non-avian dinosaurs.

Debate has raged over whether this mammalian group first appeared before or after the great extinction event 66 million years ago. Now that debate may finally be settled.

There is plenty of evidence in the fossil record placing mammal ancestors in the time of the dinosaurs. But there is no direct fossil proof that placental mammals were among these early, furry pioneers which walked with dinosaurs.

In fact, placental mammals are only found in rocks younger than 66 million years old, suggesting they evolved after the asteroid impact.

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Placental mammals (including humans, elephants, cats, rodents, etc.) have an organ, the placenta, which develops while in the womb and channels both nutrients and waste products between the foetus and the mother. This is a key feature setting placental mammals apart from the other two major living mammal groups – monotremes and marsupials.

Using advanced genetic analysis and a new statistical approach, researchers from the University of Bristol in the UK and the University of Fribourg in Switzerland have managed to “wind back” the “molecular clock” to pinpoint the evolutionary origin of placental mammals.

Their results are published in the journal Current Biology.

In their paper, the palaeobiologists reveal that their statistical model suggests placental mammals first appeared before the extinction of the non-avian dinosaurs.

“We pulled together thousands of fossils of placental mammals and were able to see the patterns of origination and extinction of the different groups,” says lead author Dr Emily Carlisle from Bristol. “Based on this, we could estimate when placental mammals evolved.”

The research is based on the idea that levels of molecular variation can be used to estimate the evolutionary divergence of different species. The analogy is that of a “molecular clock” which ticks at a constant rate leading to changes in DNA and protein sequences across different organisms.

A statistical model known as a Bayesian Brownian Bridge (BBB) was used in the study. This technique takes the present-day diversity of a group of organisms as a given, and plots many random paths backward to estimate the trajectory of different lineages back to a common ancestor. It also takes into account the established fossil record.

This is in contrast to other statistical models which require estimation of speciation and extinction processes.

“The model we used estimates origination ages based on when lineages first appear in the fossil record and the pattern of species diversity through time for the lineage,” explains co-author Daniele Silvestro from the University of Fribourg. “It can also estimate extinction ages based on last appearances when the group is extinct.”

Direct fossil evidence represents a tiny fraction of the life that once existed. Statistical techniques such as these help to fill in the gaps in our understanding of evolutionary paths for different lineages.

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This “molecular clock” analysis suggests that the first placental mammals appeared in the Late Cretaceous, near the Cretaceous-Paleogene (K-Pg) mass extinction 66 million years ago.

In fact, the study shows that several groups within placental mammals – Primates (the group that includes humans), Lagomorpha (rabbits and hares) and Carnivora (dogs and cats) – all emerged before the mass extinction, meaning our placental ancestors were mingling with dinosaurs.

It is believed that, after this initial emergence of placental mammals, the group was able to diversify rapidly following the extinction of dinosaurs which would have led to lessened competition.