A Pivotal Human Ancestor Walked With Dinosaurs, Study Finds

The question of whether the essential characteristics that characterize placental animals like us evolved in our ancestors before or after the extinction catastrophe that exterminated the dinosaurs has been a source of much controversy.

Following an investigation by academics from the University of Fribourg in Switzerland and the University of Bristol in the UK, that argument may now have finally been resolved.

Prior to the 66 million-year-old, dinosaur-killing Cretaceous-Paleogene (K-Pg) mass extinction, no conclusive placental mammal remains have been discovered. However, molecular clock evidence from the fossil record reveals that the lineage dates back even earlier, to the period of the dinosaurs.

In order to identify the common ancestors of species, analyses of molecular clock data 'wind back' genetic changes that happen continuously through time.

Researchers have demonstrated how the earliest placental mammals most likely first appeared during the Cretaceous period, briefly coexisting with dinosaurs thanks to the use of a novel statistical analysis technique.

According to Emily Carlisle, a paleobiologist at the University of Bristol, "we gathered thousands of fossils of placental mammals and were able to see the patterns of origination and extinction of the different groups."

We could estimate the time of placental animals' evolution based on this.

The researchers' model also demonstrates that more contemporary lineages of placental animals didn't begin to appear until after the asteroid impact. Therefore, it's probable that after the dinosaurs (and a large number of other species) were extinct, the conditions were better for diversification.

Based on a Bayesian Brownian bridge model, the ages of clades—groups of creatures having a common ancestor—were calculated. This kind of statistical model uses probability to identify evolutionary trends over time spans in the absence of direct evidence.

The researchers calculate that 21.3 percent of the 380 placental mammal groups in their dataset may have a Cretaceous origin.

This comprised the ancestors of dogs, cats, rabbits, and hares, as well as the primates. Furthermore, the simulations agreed well with other molecular clock findings that point to equally early origins for placental animals.

"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 evolutionary scientist Daniele Silvestro from the University of Fribourg in Switzerland. Based on the group's last sightings, it may also calculate the age of extinction.

The team contends that, in cases when there are few fossils available, the model utilized here is more reliable than utilizing genomic data or fossil records to determine a species' evolutionary routes.

It's probably not unexpected that these placental mammals don't show up in the record in their earliest forms as relatively few creatures ever attain fossil status since preserving an organism as a fossil requires a very specific set of circumstances.

The researchers are now hopeful that additional studies will be able to use the model they have established. The outcomes obtained by this statistical technique should get better as more work is done on organism categorization and digitizing fossils.

According to University of Bristol paleobiologist Phil Donoghue, "By examining both origins and extinctions, we can more clearly see the impact of events like the K-Pg mass extinction or the Paleocene-Eocene Thermal Maximum (PETM)."

The research has been published in Current Biology.