Animal of the Week, February 12, 2013 -- Darwin Day

A depiction of the last common ancestor.

If asked whether I'd ever do a mythical AOTW, I’ve always answered “no”. No dragons, no unicorns, no griffins or minotaurs. And while the Androscoggin beast, supposed big cats in the UK, and yetis have had mentions over the years, all were based on real animals. But this week’s animal breaks the only rule of AOTW: it's a work of science fiction, created by phylogenetic wizardry, and lacking a name. It’s the last common ancestor of all placental mammals (the 5000 species or so that give birth after a placenta nourished gestation, rather than lay eggs (platypuses and echidnas) or raise young in pouches (marsupials)).

Using a large database of morphological characteristics of living and fossil mammals, genetic data, and dates from fossil record, research published in the journal Science back-modelled the evolution of mammals since their divergence and determined that the last common ancestor of anteaters, cows, camels, elephants, humans, and whales was a small insectivorous shrew-like thing living about 64.85 million years ago (give or take 100 000 years). The latest estimates for the asteroid impact that did for the dinosaurs (those that aren't birds) is 66.04 million years ago. Researchers had previously thought the splits between major groups of placental mammals happened while dinosaurs still walked the earth. The new research shows that these groups probably split in the first few hundred thousand years after the extinction of the dinosaurs. This is not to say that other groups of placental mammals weren't alive with the dinosaurs, but that they went extinct too, without leaving any descendents, perhaps in the same event.

We already knew that the animal would be quite shrew-like, because most fossil mammals from the time of the dinosaurs were. But according to the analysis tracing back the evolution of thousands of characters described in extant and extinct mammals, the animal weighed 6-245 g, ate insects, and likely gave birth to one hairless baby. Of course, this is science, and it could all be proven to be wrong. But the last common ancestor definitely did exist, and maybe this is what it was like and when it lived.

February 12 is Darwin Day, honouring the man whose work laid the foundations for the science of evolutionary biology, so I think this wonderful story is a fitting AOTW to mark the occasion.