The common ancestor of all mammals

I have been fascinated by the fact that evolution tells us that if we could journey back in time, then all of our ancestors start to merge, with some surprising results. I wrote a series of posts exploring this topic some years ago and pointed out some surprising (to me at least) results.

It turns that we only needed to go back as far as 50 CE or so to find that everyone on Earth right now had at least one common ancestor and that if we go back to around 2,000 BCE then everyone who lived then (and whose line didn’t die out entirely) are the common ancestors of everyone who lives now. In other words, all of us living today had the identical set of ancestors who lived as recently as 4,000 years ago,.

common ancestorThis means that we are all cousins and makes all our tribal divisions seem so meaningless and trivial

But you can go back even further and then you find that entire species start merging with other species to have a common ancestor and at some point, on the way to the final destination of the common ancestor of every living organism, you will arrive at the common ancestor of all the mammals.

It has long been thought that the common ancestor of mammals, that came into its own after the dinosaurs were killed off by the asteroid or comet that collided with the Earth about 66 million years ago was a small shrew-like creature.

Ed Yong points to new paper that uses DNA of the species living today to try and construct a more reliable picture of what that ancestor might have looked like and they come up witht something that would look like the animal in the picture on the right.

The critter turned out to be a tree-climbing, furry-tailed insect eater that weighed between 6 and 245 grams. It gave birth to blind, hairless young, one at a time. Its brain was highly folded, and it had three pairs of molars on each jaw.

The researchers analysed more than 4,500 anatomical traits — almost ten times as many as any previous study. These included traits from 86 living and extinct mammals.

The analysis, which is published in Science, 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.

This creature must have been around after the supercontinent of Gondwana split into today’s southern land masses, so its descendants must have swum or otherwise travelled over long distances to explain the wide distribution of placentals today.

So say hello to our great-great-great-…-great-grandparent.


  1. Timothy says

    “This means that we are all cousins and makes all our tribal divisions seem so meaningless and trivial ”

    Imagine a world where this became front-page news … and people began acting accordingly.

    Sigh …

  2. AsqJames says

    That was my first thought too, but then I thought about how some families I know treat each other (sad face)!

  3. busterggi says

    Fascinating but I refuse to invivte 8,000,000,000 relatives to my daughter’s wedding. Even with a discount.

  4. invivoMark says

    I have a hard time believing that someone from 50CE had descendants that traveled to every single corner of human civilization – from Tierra del Fuego to the bushmen of the Kalahari, to every single minor pacific island, to Japan and Mongolia and Nova Scotia. Is that really feasible? Consider that, nobody in Europe ever traveled to North America for thousands of years. Consider that the Bering land bridge was gone at least 6,000 years ago. I just don’t see how it’s possible.

    Furthermore, if this person were a male, wouldn’t that make him Y-chromosomal Adam? And yet we know that Y-chromosomal Adam lived 60,000 years ago. Or if this person was a female, she’d be Mitochondrial Eve, who we know lived almost 200,000 years ago. So doesn’t that definitively rule out the idea of a common ancestor less than 3,000 years ago?

  5. Reginald Selkirk says

    invivoMark: outsiders have now been to all of those places, so your question boils down to: are there any natives of those places remaining who do not have some non-native ancestry?

  6. invivoMark says

    Obvious answer: yes.

    Less obvious answer: recent expansion of travel wasn’t considered in the calculation Mano is referring to. The calculation was based on simple population genetics equations, based on random mating, and adding limiting factors to account for travel among cities, countries, and continents. A limitation of this method is that it appears to have considered all travel of equivalent order (i.e., city-to-city, or continent-to-continent) to be equally likely to and from any population.

    My objection based on mtEve and ycAdam is incorrect, however. Upon further reflection, mtEve is related to every modern woman through a strictly maternal line, and ycAdam to every man through a strictly paternal line. The MRCA of all humans can be related through lines that contain both men and women, and is therefore expected to be far more recent than mtEve or ycAdam.

    I blame a lack of sleep for my muddled thinking. I should know this stuff.

  7. Jared A says

    invivoMark: It’s disappointing to me that these results are from a simulation instead of actual data. Do you know if anyone has figured out (from actual data) the probability distribution of common ancestry? I’m thinking of something like the genetic version of a two-point correlation function. In other words, what is the probability that a random person shares a common ancestor with another random person within a certain number of years or generations? I think this is an interesting question and probably more meaningful than the “MRCA” question. It also would tell you how far back to go to get a vanishingly small fraction of people on earth that don’t share common ancestors.

  8. invivoMark says

    Various genealogy websites seem to offer such probabilities, based on tests that they are invariably trying to sell you. I can’t vouch for their accuracy.

    I’m sure someone has calculated the odds of two random people sharing a common ancestor n generations prior, but I can’t find it over the noise of genealogists trying to sell products.

    It’s worth noting that the paper Mano references acknowledges the shortcomings to their method:

    “Several factors could cause the time to the true MRCA or IA point to depart from the predictions of our model. If a group of humans were completely isolated, then no mixing could occur between that group and others, and the MRCA would have to have lived before the start of the isolation. A more recent MRCA would not arise until the groups were once again well integrated. In the case of Tasmania, which may have been completely isolated from mainland Australia between the flooding of the Bass Strait, 9,000–12,000 years ago, and the European colonization of the island, starting in 1803 (ref. 13), the IA date for all living humans must fall before the start of isolation. However, the MRCA date would be unaffected, because today there are no remaining native Tasmanians without some European or mainland Australian ancestry.”

    I was curious, so I looked up how many isolated populations there could be. Turns out, there are quite a few!

  9. sc_770d159609e0f8deaa72849e3731a29d says

    What about the monotremes- platypuses and echidnae- egg-laying milk producers? where do they fit in?

  10. lsamaknight says

    Gotta agree with the googlemess @ 7. This article only seems to be talking about the placental mammals and excludes both marsupials and monotremes. Including those would push the date for the MRCA of mammals back considerably further than the K-T boundary considering that they’ve got fossil remains of the platypus ancestor dating back to Jurassic.

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