Credit: Carl Buell
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A reconstruction of the ancestral placental mammal
Credit: Carl Buell

Meet the Ancestor of Every Human, Bat, Cat, Whale and Mouse

The blue whale—190 tonnes in weight and beautifully adapted for swimming—is a placental mammal. The mammal bit means that mothers nourish their babies with milk after they’re born. The placental bit means that mothers nourish their babies via a placenta before they’re born—an organ that allows them to exchange oxygen and nutrients without also swapping blood.

The bumblebee bat—1.5 grams in weight and beautifully adapted for flying—is also a placental mammal. So are you. So is a bear, an anteater, a giraffe and a squirrel. Also: armadillos, rhinos, rabbits, manatees, and pangolins.

All of these creatures, in their wondrous array of shapes and sizes, evolved from a small, unassuming, scurrying insect-eater that lived a few hundred thousand years after the apocalypse that finished off most of the dinosaurs.

A team of US scientists have now reconstructed what this ancestral placental was like, to an extraordinary level of detail. They have predicted how much it would have weighed, the number of molars in its jaws, the shape of its sperm, and the path that its carotid artery took up its neck. None of this comes from a fossil of the creature itself. Instead, the predictions are based on 80 of its descendants, including some that are still alive and others that joined it in extinction. To find out more details about the results (and what they mean about when placentals evolved) have a look at my coverage for Nature News.

Meanwhile, I wanted to publish the explanation that one of my sources—Olaf Bininda-Emonds from Oldenburg University—sent me, which explains just how much work went into this.

To do their analysis, the team had to score the skeleton of 86 different species according to more than 4,500 anatomical traits. Think of an enormous table that they had to fill in. Here’s Bininda-Emonds on what that took:

I find the study to be absolutely stunning. The data matrix of characters that they’ve assembled is jaw-dropping and, when combined with DNA sequence data, undoubtedly provides one of the best estimates of evolutionary relationships within placental mammals to date.

Just to put into perspective what an incredible amount of work coding over 4,500 characters for nearly 90 different species is, I once looked at a “mere” 200 characters for 35 species (for my Masters project) and it was six weeks straight of sitting in various natural history museums for 8+ hours a day. Incredibly arduous and not really that exciting. Kinda dusty too. Now scale that up by a factor of over 20!

What’s lost in the results, however, was the effort in coming up with the 4,500 characters in the first place. I’ve heard rumours of how it took years for them just to come up with the character list itself. That seems hard to believe, but you have to remember that they had to come up with a list of characters and character definitions that fits everything from a bumblebee bat to a blue whale and all the weird and wonderful forms in between (e.g., beavers, elephants, seals, sloths, meerkats, even humans). That’s a lot of diversity to try and summarize.

To make matters worse, they often had to do a lot of detective work because the same structure could often go under a half dozen aliases, with the name/definition often being peculiar to a particular taxonomic group. So, there was a lot of science going on here as well in trying to decide if a particular flange on the top of the femur in something like a walrus was evolutionarily the same structure as a ridge in a similar position on the femur of an aardvark! And, if so, then what should they call it?