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    The Oldest Known Common Ancestor of Humans Looked Like a Chill Rat-Possum Thing

    Written by Derek Mead

    An artist's rendering of the little mammal in question. Credit: Carl Buell

    Man's common ancestor with the rest of the mammal tribe may have been a small rat-like insectivore with a really long tail. Say hello to one of your oldest relatives, named Protungulatum donnae.

    Creationists love to ramble on about how humans couldn't have evolved from monkeys, which is true. Humans and the rest of the apes all speciated from common ancestors: the species that, for various reasons, has evolved into two distinct species. If you go back through the mammalian tree, you'll find the point at which the branch that led to horses and the branch that led to lions split, and if you go way back, you'll find the first common ancestor mammals share, or the point at which mammals essentially began to exist.

    While the new find isn't exactly that point, it is perhaps the point at which placental mammals, which is the vast majority of mammals today, split off from marsupials and egg-laying mammals. After studying the mammalian tree for six years, a group of researchers at Stony Brook University have sorted out what they believe is the ancestor of most modern mammals. Furthermore, the research syncs up with the massive extinction event at the end of the Cretaceous, which has long been posited as the point at which dinosaur die-offs allowed for mammals to flourish.

    "Analysis of this massive dataset shows that placental mammals did not originate during the Mesozoic," lead author Maureen O'Leary, an associate professor at Stony Brook University, said in a release. "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."

    The point at which placental mammals arose has been the source of debate for some time now. Research has suggest the first mammals arose as long as 100 million years ago, and that by the time of the Cretaceous extinction, that lineage had split into about 20 different genetic lines. But fossil evidence of modern placental animals has not been found in that period. This new research, published in Science, suggests that P. donnae was the first animal to have all the characteristics of placental mammals, which would place the rise of the bulk of modern mammals in the post-dinosaur period.

    "There are over 5,100 living placental species and they exhibit enormous diversity, varying greatly in size, locomotor ability, and brain size," co-author Nancy Simmons of the Department of Mammalogy at the American Museum of Natural History said. "Given this diversity, it's of great interest to know when and how this clade first began evolving and diversifying."

    Utilizing 4,500 points anatomic data each from 46 living and 40 fossil species, along with data from an online data repository called MorphoBank, the team developed a tree spanning the evolution of mammals. They found that P. donnae, which weight at most half a pound and is from a fairly obscure genus, showed the first signs of placental development about 400,000 years following the Cretaceous extinction. Within a couple million years after that–possibly aided by the empty niches left by the dinosaurs–mammalian diversity exploded.

    The National Science Foundation, the project's biggest backer, funded the research through its Assembling the Tree of Life program, whose goal is to discover the evolutionary origins of everything on Earth, and which relies heavily on data analysis of the complicated fossil record. This finding is perhaps the most fascinating result of morphological and evolutionary research becoming more and more data driven, partially because it goes a long way towards answering where we all came from. But how do you feel to know that you, over millions of years, have descended from a fuzzy little bug eater? I, for one, am pretty chill with it.