A Blog by Ed Yong

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?

18 thoughts on “Meet the Ancestor of Every Human, Bat, Cat, Whale and Mouse

  1. Strangely enough, I’ve wondered whether a thing like this could ever be done, and now that it has, it’s very exciting! I assume that figuring out a hypothetical common ancestor could be done for a variety of groups like fish, insects, and even ornithurine birds!

    I await the discovery of this little furball in the future.

  2. Is that timing right? It was my understanding that the last common ancestor of all placental mammals was around 105Mya (this is coming from the Ancestor’s Tale, by Richard Dawkins).

    Was the data he presented in that book incorrect?

    Aside from that, though, this is a VERY excellent and impressive reconstruction. :)

  3. Alex, if you look at the Nature News article link you can read the following:
    “The analysis, which is published in Science1, confirms that the placentals diversified a few hundred thousand years after the (non-avian) dinosaurs went extinct, so groups such as rodents and primates never shared the planet with the prehistoric reptiles. This conclusion is backed up by the fact that no one has ever found fossils of placental mammals from before the extinction of the dinosaurs 65 million years ago — but it contradicts genetic studies that put the group’s origin at around 100 million years ago.”

  4. How does this reconcile with the molecular data, which I think indicates that the splitting of the various modern orders occurred deep in the Cretaceous or even earlier?

  5. Apologies if this is a double post, but I tried a few minutes ago and it didn’t appear.

    The term ‘placental’ can be used in three more or less inclusive senses:
    1. Eutherian, i.e. more closely related to primates than to marsupials.
    2. As Ed put it, “mothers nourish their babies via a placenta”.
    3. Belonging to the crown group Placentalia, i.e. descended from the latest common ancestor of living eutherians.

    In sense 1, The oldest known eutherian is Juramaia, our 165-million-year-old Jurassic mother. For about the first half of their history, eutherians gave birth to comparatively small young, so they did not have a pregnancy similar to most mammals today.

    Sense 2 is hard to diagnose: according to the fossil record, the history of mammals consists mainly of teeth breeding to produce slightly modified descendant teeth. I don’t think a fossil placenta has ever been found. However, a few late Cretaceous eutherians had relatively wide hips and other characteristics suggesting the foetus grew in the womb. They are generally thought to be placental in this sense.

    O’Leary et al are dealing with sense 3. Presumably their analysis rejects the theories that Cretacous eutherians such as Protungulatum, Zalmbdalestes, and Cimolestes have affinities with ungulates, rodents and carnivores respectively. The New York Times (http://www.nytimes.com/2013/02/08/science/common-ancestor-of-mammals-plucked-from-obscurity.html) suggests the authors named a younger species of Protungulatum as the most recent common ancestor, but that is probably not exactly what they said.

    Alex & Jerrold:
    “How does this reconcile with the molecular data, which I think indicates that the splitting of the various modern orders occurred deep in the Cretaceous or even earlier?”
    It doesn’t. The question is discussed here, starting about halfway down the page http://palaeos.com/vertebrates/eutheria/eutheria2.html. Palaeos favours the ‘explosive model’ of a radiation around the start of the Tertiary, consistent with the new O’Leary et al paper.

    Dawkins must have based the mammalian chapters of the Ancestor’s Tale on a molecular clock. He may have revised his opinion in the 10 years since.

  6. Proto-mammals had to come from something. If they were derived from reptiles, amphibians or whatever, the there had to be an overlap beginning before the extinction of dinosaurs, or they arose from something else following or during said extintion. What do genetic studies on the Platypus say?

  7. Sagitta,
    Thank you for the link to Palaeos.

    C.H. Hood,

    Mammals were not derived from reptiles. Soon after the amniotic egg developed within vertebrates, amniotes split into two groups: the sauropsids, marked by two openings in the palate, and synapsids, marked by an opening behind the eye. Mammals developed within the synapsid lineage. Extant mammals include the placentals, the marsupials and the monotremes. The platypus is a member of the latter group.

  8. Proto-mammals came from Synapsida (via therapsids), which could be considered primitive reptiles, derived before actual dinosaurs came along (though I think the archosaurs from which dinos came predated synapsids).

  9. evodevo,
    Synapsida are NOT “primitive reptiles”. Reptiles and synapsida are the two big divisions in the amniotes. Archosaurs are a derived group of reptiles (or sauropsids, to be more accurate). Dinosaurs (including birds) are a clade within archosaurs. mammals are a clade within synapsids.

  10. I never really liked mammals that much… only a few, such as cats, bears, cows, seals, giraffes, and a few more. I agree with this thing being ugly. Just wondering, what was the ancestor of all insects? You know, like the animal that evolved into the insects? Anyway thanks for reading this and if you know the answer please tell me!

  11. In the end, everything is related to everything else, right down to the primordial ooze. Cool! I like ooze.

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