In today's issue of the journal Science, a case is made of how, all things considered, the oldest common ancestor that all placental mammals share is a rat-looking creature which emerged as a self-standing species roughly 200,000 years after the dinosaurs had become extinct.
The researchers who have embarked on this project of tracing down the oldest human ancestor have worked on the assumption that the reign of dinosaurs came to an end following the planet's being hit by a massive asteroid, and not because of factors having to do with climate and food availability.
Thus, they maintain that the mammal now labeled as the common ancestor of all placental animals made its appearance roughly 200,000 years after the dinosaurs had fallen off the biodiversity map.
Seeing how several other animal species became extinct together with the dinosaurs, this rat-like creature found itself able to spawn as many placental mammals as it saw fit, and soon enough (i.e. in about 200,000 – 400,000 after the dinosaurs had disappeared) others of its kind made their way into natural ecosystems worldwide.
Daily Mail quotes Maureen O'Leary, a specialist now working as an associate professor in the Department of Anatomical Sciences in the School of Medicine at Stony Brook University, who commented on these findings as follows:
“Species like rodents and primates did not share the Earth with non-avian dinosaurs but arose from a common ancestor - a small, insect-eating, scampering animal-shortly after the dinosaurs demise.”
In order to piece together this new so-called tree of life for placental mammals, the researchers who have taken part in these investigations have analyzed information having to do with both the genetic characteristics, and the physical traits of such animals.
Thus, they argue that the oldest common ancestor for all placental mammals was basically a small creature with a rather lengthy tail and with a voracious appetite for insects.
“There are over 5,100 living placental species and they exhibit enormous diversity, varying greatly in size, locomotor ability, and brain size. Given this diversity, it's of great interest to know when and how this clade first began evolving and diversifying,” stated Nancy Simmons, a curator at the American Museum of Natural History.