ADVERTISEMENT
  About the SA Blog Network













Observations

Observations


Opinion, arguments & analyses from the editors of Scientific American
Observations HomeAboutContact

Meet the Last Common Ancestor of Bats, Whales, Sloths and Humans

The views expressed are those of the author and are not necessarily those of Scientific American.


Email   PrintPrint



Placental ancestor

Reconstruction of the hypothetical ancestor of all placental mammals. Image: Carl Buell

They may run, swim or fly. They may weigh less than a penny or more than a dozen school buses. From humans to whales to bats, the placental mammals—so named for the placenta that nourishes the fetus during development—are mind-bogglingly diverse. (The placental mammals are one of three major groups of mammals; the other two are the egg-laying monotremes and the pouched marsupials.) For years researchers have been attempting to piece together when the placentals originated and when the group’s modern orders, such as the primates and the bats, first emerged. Now a major new analysis of thousands of anatomical features of modern and extinct mammals, as well as molecular sequences from living species, is helping them to do just that. The study also hints at what the ancestral placental mammal—the one that ultimately gave rise to creatures as disparate as tree sloths and sea lions–looked like.

Previous attempts to reconstruct the evolutionary history of mammals yielded conflicting scenarios. Fossil evidence suggested that the placentals burst onto the scene shortly after a massive asteroid slammed into the earth around 65 million years ago and snuffed out the dinosaurs. Studies that instead used molecular sequence data to get at the question of when placentals originated and diversified painted a different picture, indicating that the group appeared as early as 100 million years ago, when dinosaurs were still thriving. This date is far older than the oldest known placental fossils, but it helped to explain the diversification of the group: the supercontinent Gondwana was fragmenting at that time and the breakup  would have provided opportunities for populations to become separated and evolve in isolation into new forms.

The new study, which looked at more than 4,500 traits in 86 fossil and living mammal species, controverts the early origin model, concluding instead that the placentals originated after the mass extinction event, with the first members of modern placental orders evolving some two million to three million years later–after the continental breakup. Maureen O’Leary of Stony Brook University and her collaborators describe their findings in the February 8 Science.

Perhaps the coolest part of the paper is the bit where the authors reconstruct the characteristics of the hypothetical placental ancestor—a tree-climbing, insect-eating beastie that weighed between 6 and 245 grams and gave birth to one hairless baby at a time, among other fascinating details. I’d love to see what other hypothetical ancestors look like–last common ancestor of chimps and humans, anyone?

 

 

 

 

Kate Wong About the Author: Kate Wong is an editor and writer at Scientific American covering paleontology, archaeology and life sciences. Follow on Twitter @katewong.

The views expressed are those of the author and are not necessarily those of Scientific American.





Rights & Permissions

Comments 10 Comments

Add Comment
  1. 1. Mythusmage 7:12 pm 02/7/2013

    I bet on the genes. Fossils of a certain age only maan you have fossils of a certain age, not that you have the oldest specimens.

    Link to this
  2. 2. American Muse 8:01 pm 02/7/2013

    An overactive imagination. perhaps?

    Link to this
  3. 3. RSchmidt 11:02 pm 02/7/2013

    I think the tree of life is far too bushy to be able to claim you have the last definitive common ancestor of any two species. You may have something that looks similar. You can hypothesis on the genome. Genes do not fossilize well.

    I would like to see the ancestor of bilateral and radial symmetrical organisms, its probably a worm. The ancestor of bats and primates would be interesting, probably like a cross between a lemur and a flying squirrel. The common ancestor of Creodonta and Carnivora would be cool.

    Link to this
  4. 4. RSchmidt 11:07 pm 02/7/2013

    @Mythusmage, well I think that may be an oversimplification. You can certainly claim that you have the oldest known ancestor based on morphology and radiological dating. It may not be THE oldest but that’s not the point.

    Link to this
  5. 5. RSchmidt 11:10 pm 02/7/2013

    Great picture by the way. Carl does amazing work!

    Link to this
  6. 6. mipakeli 2:30 am 02/8/2013

    It’s truly a rat race on Earth.

    Link to this
  7. 7. Bill_Crofut 3:45 pm 02/8/2013

    Re: “…[T]he placental mammals—so named for the placenta that nourishes the fetus during development—are mind-bogglingly diverse….Previous attempts to reconstruct the evolutionary history of mammals yielded conflicting scenarios.”

    Prof. W. R. Thompson would seem to have addressed the diversity/conflict issue nearly 60 years ago:

    “What such cases like those of anatomical ‘convergence’ and general homology actually demonstrate is that there are large numbers of organisms differing considerably in the details of structure but constructed on the same fundamental plan. However, this is no proof of descent from one original ancestor of this anatomical type. This itself requires proof….What we call the natural system of classification is a proof of evolution since it can only be explained as a result of evolution….The argument specifically implies that nothing is exempt from this evolutionary process. Therefore, the last thing we should expect on Darwinian principles is the persistence of a few common fundamental structural plans. Yet this is what we find.”

    [1956. Introduction. In: Charles Darwin. Origin of Species. Everyman Library No. 811. London: J. M. Dent and Sons. Reprinted with permission. Evolution Protest Movement. 1967. NEW CHALLENGING ‘INTRODUCTION' TO THE ORIGIN OF SPECIES. Selsey, Sussex: Selsey Press Ltd., pp. 11-12, 13]

    Link to this
  8. 8. noretreat 7:46 pm 02/8/2013

    Why just ONE ancestor for all? Could the placenta have evolved naturally in two or more diverse species?

    Link to this
  9. 9. alan6302 9:20 pm 02/8/2013

    I’ll be happy if they can find a link between humans and hominids. A link that does not require genetic engineering.

    Link to this
  10. 10. David Marjanović 3:13 pm 02/11/2013

    Perhaps the most important part of this paper is the fact that is that the “paper” behind the paywall is, in reality, just an extended abstract. It presents the showiest results and has all the pretty pictures, but that’s it. The entire “materials and methods” section is in the allegedly supplementary information.

    In a comical turn, this “supplementary” information has its own supplementary information (“appendices S1 to S4″), which isn’t even hosted on sciencemag.org, but on morphobank.org!

    In other words…
    This is an open-access paper!

    Simply go here, cackle madly, and click on “download supplement”!

    I bet on the genes.

    Molecular divergence dating analyses can’t produce dates on their own. They have to be calibrated… from the fossil record. And that’s easy to do wrong (Brochu 2004, me & Laurin 2007).

    Fossils of a certain age only maan you have fossils of a certain age, not that you have the oldest specimens.

    Sure, but the fossil record isn’t random either.

    An overactive imagination. perhaps?

    No. The paper shows in detail how the authors reconstructed the first placental from their phylogenetic tree. Making such reconstructions is actually an integral part of the tree reconstruction process, it’s just hidden inside the computer and not normally brought out; but this time, the authors had so many characters in their dataset that you can basically read the entire animal out of the tree. Just the fur colors are conjecture.

    I think the tree of life is far too bushy to be able to claim you have the last definitive common ancestor of any two species.

    Exactly, that’s why the first placental can only be reconstructed.

    The common ancestor of Creodonta and Carnivora would be cool.

    …Especially because there’s no evidence that such a thing as Creodonta even exists. Oxyanidae and Hyaenodontidae are simply the only two groups that haven’t been taken out yet. While Oxyaenidae shows up in the Paleocene of North America, Hyaenodontidae originated in Africa…

    The ancestor of bats and primates would be interesting, probably like a cross between a lemur and a flying squirrel.

    No, bats and primates are not sister-groups, so this ancestor would look a lot like the first placental.

    Great picture by the way. Carl does amazing work!

    Oh yes.

    It’s truly a rat race on Earth.

    Oh no. Rats are rodents, they have completely different heads.

    Why just ONE ancestor for all? Could the placenta have evolved naturally in two or more diverse species?

    It did, in fact – there are marsupials and lizards with a placenta, for example. Placentalia isn’t somehow defined by the placenta; the placentals have lots of other features in common, too – so many that a single common origin is by far the simplest explanation, as this paper once again confirms.

    I’ll be happy if they can find a link between humans and hominids.

    Uh, Hominidae is defined as “the family that contains Homo“.

    (“Family”, though, is not defined. That’s why the size of Hominidae has fluctuated so widely over the decades – for some, it contained only Homo and Australopithecus, for others all apes are hominids, and you can find everything in between in the literature.)

    Link to this

Add a Comment
You must sign in or register as a ScientificAmerican.com member to submit a comment.

More from Scientific American

Scientific American Holiday Sale

Black Friday/Cyber Monday Blow-Out Sale

Enter code:
HOLIDAY 2014
at checkout

Get 20% off now! >

X

Email this Article

X