Mammals didn’t inherit the Earth - they took it when the time was right.
This furry, bug-eating critter might not look very formative and, in fact, a 99¢ mousetrap would make short work of him. Yet the lineage of mammals to which he belongs were able to survive the famous extinction event 66 million years ago when an asteroid wiped out the non-avian dinosaurs, along with 75% of all the plants and animals on Earth. It’s called Protungulatum donnae, and this artist’s depiction of the creature closely resembles what may well have been the last common ancestor of all extant placental mammals, which includes everything from cats to whales and mice to men. That’s quite an achievement. Though, to be fair, that also makes it indirectly responsible for the band Menudo, Pauly Shore, the Pontiac Aztec, Windows Vista, Crystal Pepsi, the Star Wars prequels, Steak-umms, Ronco’s GLH Formula spray-on hair for balding men and a million other things for which it will never, ever be forgiven.
Placental mammals have actually been around for 160 million years, as long as the dinosaurs, but it’s thought that they remained small, shrew-like creatures until the dinosaurs disappeared, which opened up niches in the environment allowing them to thrive. Researchers have been trying for years to pinpoint the last common ancestor of modern placental mammals and establish a timeline for when the modern orders, such as bats and primates, emerged. Protungulatum donnae emerged 200,000-400,000 years after the extinction event, quickly branching out and evolving into various lineages of mammals, many of which are still around today. Maureen A. O’Leary of Stony Brook University on Long Island, led the team which compared thousands of data points using anatomical features from modern animals and fossils. “It only took 2 or 3 million years after the extinction for the first members of modern placental orders to appear,” said Dr. O’Leary. The mammals took over so fast and with such profusion that researchers have started calling it the “explosive model of mammalian evolution.”
Even with this exhaustive and detailed analysis, a glaring issue still needs to be worked out. Using molecular data alone, the emergence of Protungulatum donnae is estimated to be around 100 million years ago, not 66 million years as most research predicts. If the molecular data are correct, the mammalian explosion started when the dinosaurs were still around. Yet, there are no fossils from that time to support this idea. Fossil hunters are on the prowl and the geneticists are currently looking for flaws in their methodology. Rectifying the disagreement between the fossil/anatomical data and the molecular evidence is the subject of vigorous debate, so even though they’re very certain of what your long-gone ancestors looked like, whether or not they actually walked with the dinosaurs is still a bit of an open question.