The study of the origin of whales has undergone so much change during my own lifetime that it's sometimes hard to keep up. When I was very young, Basilosaurus was the fossil whale representative, but being that it was already a whale it didn't solve the problem of whale origins. Newer discoveries of older cetaceans in Asia like Pakicetus, Ambulocetus, Kutchicetus, and Rodhocetus provided a group of transitional types that stunned vertebrate paleontologists, but a major debate loomed over these fossils. While it first appeared that these"walking whales" were closely related to the carnivorous ungulates called mesonychids, genetic analysis, the presence of a "double-pulley" ankle bone, and the distribution of weight across the toes suggested that whales evolved from artiodactyl ancestors, their closest living relatives being hippos. There are still major problems, however, and the debate is as vigorous as ever. Do cetaceans belong within the Artiodactyla, supporting the construction of the Cetartiodactyla to contain both groups in one superorder? Or do cetaceans and artiodactyls (including hippos) belong to sister superorders, the Cetartiodactyla representing the grouping of the two together without nesting cetaceans within the artiodactyla? The fact that the fossil trail of hippos and cetaceans don't mesh well has further muddied the waters of the issue, but in a new paper presented today in the journal Nature, a fossil creature called Indohyus is proposed as a representative of what the earliest whale ancestors might have been like.
As it stands now, the earliest known cetaceans are about 50 million years old, the best known (or at least most widely-cited) being Pakicetus, an animal that has undergone something of a makeover since it was discovered in 1983. Early representations based upon partial material made Pakicetus look like a chubby seal with a tail, but more complete finds revealed that Pakicetus was still more terrestrially-adapted despite its aquatic habits, illuminating a key point in the transition into the water. This presents a bit of a problem when we compare the earliest known cetaceans to the earliest known hippopotimids; the earliest known members of the hippo family are about 15 million years old with the earliest known Asian representatives being about 6 million years old. Genetic analysis may continue to support a close relationship but the fossil record in this case has yet to provide sufficient clues to show us how cetaceans are related to hippos, and Indohyus may be a representative of a better candidate for a sister group to cetaceans, the Raoellidae.
The primary character that, according to the new research, links Indohyus to cetaceans is the presence of an involucrum, a thickening of the tympanic bone in the ear that has been thought to be characteristic of cetaceans. Given this, the study argues, the Raoellidae should be grouped within the Cetacea or be the sister group to it, the position of sister group being preferred as grouping within the Cetacea seemed to cause instability in the phylogenetic trees. According to the cladogram presented, then, the Raoellidae and Cetacea would comprise sister groups that would together form a sister group to all other artiodactyl groups. (Indohyus could not be an ancestor to the Cetacea as it is slightly younger than Pakicetus and Raoellids temporally overlapped with the evolving cetaceans, an inferred common ancestor being absent at this point.) It's unlikely that such a distinctive trait evolved more than once so Indohyus may very well then be part of a sister group to cetaceans (although other Raoellids need to be tested), but we are still left with the question of whether both groups should be nested within the existing Artiodactyla (to make a monophyletic Cetartiodactyla) or should remain outside, in which case it might not be possible to group the two superorders under the name Cetartiodactyla. This is the area that requires more resolution, and I think Indohyus alone isn't sufficient to shake the evolutionary bush as much as might be supposed.
In order to bolster their case, however, the authors present some additional evidence that Indohyus had aquatic habits. Many aquatic animals, extinct and extant, have had their bones adapted to act as a sort of ballast, the condition being known as osteosclerosis. When the limb bones of Indohyus were studied it too exhibited this weight-increasing adaptation, which the researchers infer as meaning that it may have walked along the bottom of nearby watery habitats in much the same way that hippos do today.
Isotope data from the teeth were a little more difficult to interpret, however, as the values arrived at for Indohyus did not fall within groupings detected for terrestrial mammals or Archaeocetes like Pakicetus. Whatever it was eating remains ambiguous, although it could have been eating on land while spending considerable time in the water. The authors briefly ponder some aquatic food sources for this animal, but they note that further study of tooth wear and other factors will be needed to figure out what this animal was consuming. When contrasted to early cetaceans like Pakicetus, though, Indohyus looks more like a herbivore than a carnivore, which does have important implications for whale evolution. If Raoellids really are the sister group to Cetaceans, then perhaps the common ancestor was more of an omnivore than either a carnivore or an herbivore. Either that or the ancestor was a carnivore or herbivore and one group went in the opposite direction while another retained most of the ancestral habits, but until more fossils are found it seems difficult to tell how the two groups that both lived during the same time could have arrived at different body types and food preferences. (As a result of my recent work on primate carnivory for my term paper, as well, there are important physiological costs to switching from becoming an herbivore to a carnivore, as well as problems with disease and parasites. The switch can be made and it obviously has been in the past, but for now I wouldn't say Indohyus can be taken as being the archetype of cetaceans earlier than Pakicetus.) The hypothesis presented in the paper is that Indohyus is representative of an cetacean ancestor, but while it might provide a model for some aspects of early cetacean evolution it does present a large number of problems as well. If Raoellids really are the sister group to cetaceans, we'll have to go back further in the fossil record of Asia as cetaceans and Raoellids were clearly on different evolutionary paths during the time period being discussed.
Overall, the position of Raoellids as a sister group to cetaceans based upon Indohyus seems pretty flimsy, especially when presented in a few scant pages in Nature. Much more research is required to help resolve this issue, especially a greater number of well-preserved Indohyus specimens, hopefully one that is articulated. Indeed, the skeletal reconstruction is a composite and it appears that the comparison of the inner ear, essential to the argument being made in the paper, was based only on one specimen, so at the moment all I can really say about the hypothesis is "It's an interesting idea." The osteosclerosis in the bones of Indohyus suggests that it was spending a good amount of time in the water (and not just fleeing into the water as suggested in the paper and some popular reports), but the data from the teeth don't seem to fit in with a wholly aquatic lifestyle, so what Indohyus was doing on the land and in the water remains inconclusive. If nothing else, this is an area that merits some more research and the hypothesis could be tested by going back to Asia to try and find some older fossils closer to when Raoellids and Cetaceans would have split from each other (especially on the cetacean line), but I guess we'll just have to wait.
Media-coverage surrounding this new paper isn't helping to resolve misunderstandings about evolution, however. In the AP piece reproduced via Yahoo!News, writer Seth Borenstein can't seem to figure out just what Indohyus is. His confusion is apparent from the first line of the article;
It sounds like a stretch, but a new study suggests that the missing evolutionary link between whales and land animals is an odd raccoon-sized animal that looks like a long-tailed deer without antlers. Or an overgrown long-legged rat.
Borenstein scrabbles to lump Indohyus in with some modern animals in a feeble attempt to get people to understand the fossil find, but I can't help but wonder if such a comparison does more harm than good. I'm not defending Indohyus as the archetype for whale ancestors, but the reporting style and comparison of unique extinct animals with modern ones (we've got raccoons, rats, deer, pigs, and dogs all in the same article) obscures rather than illuminates evolution. It's this sort of description that makes it so easy for creationists to conjure up cartoons of cows with dolphin torsos in order to foster incredulity about evolution. Sure, Raoellid doesn't really roll off the tongue, but I think we need to stop assuming that the general public just won't understand what these animals were and take a paragraph or two to explain where they fit into the scheme of things rather than saying "It's a raccoon-deer-rat-thing." If we are to educate people about these discoveries we have to respect that they can understand what we're talking about if we choose our words carefully and take the time to do so, although I wouldn't expect some writers in the mass media to change tactics anytime soon.
[As Neil notes, though, Borenstein seems to have taken a cue from lead author Thewissen in using somewhat awkward animal analogies, and at least the errors aren't as egregious as that whole Leaky-gate thing from a few months ago. Given some of the comments I've seen about Indohyus in comment threads and in message boards, though, some people thing that scientists are saying that whales evolved from deer or rats, so my aggravation remains justified.]
References;
Thewissen et al. (2007) "Whales originated from aquatic artiodactyls in the Eocene epoch of India," Nature, Vol. 450, pp. 1190-1195
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I'm gonna need that paper, Brian. :-) No, seriously, before I go to Kansas in twelve hours.
Where, technically, I'll still have the interweb, but this will make great airplane reading!!!
I think the cetacean middle ear is tough to converge on. On the other hand, Indohyus is clearly NOT a whale. Like I said on my extremely brief blog post, it's odd that anything would go from vegetarian to carnivory. Instead, what we might be seeing is a sort of tri-tree with whales, hippos, and this little bugger. Primitive hippo fossils would clear up any convergences or reversals (maybe they all originally had the cetacean middle ear?).
I like that skeletal reconstruction. While in Kansas, I think I'll do a comparison drawing of known archaeocetes, including terrestrial forms. Good summary, though, Brian.
Oh, and on the media thing: I'm no fan of the Associated Press (as you might already know!), and I agree that trying to compare long-extinct primitive mammals to modern creatures does more to confuse the lay audience than anything else. So whales are related to deer? Or rats? Or hippos? No, not really. Animals were different back then. We don't always need a modern analogue.
Well, compared to the whole "calling into question the evolution of our ancestors" gaff, I'd say Borenstein's turned in solid C work this time.
To be fair his awkward smilizing seems to be inspired directly by comments from Thewissen.
Oh you beat me to it! I've been trying to write an uber-post...y'know, to establish in my own mind that my blog is more than just random gibberish...and this was gonna be my topic.
Well, sorta. I was gonna use the article to introduce creationist crap...see here. You'll see that the footnote is referencing a paper in the Creation journal.
Well, you did a much better job on this than I would have. I'll move on to plan b (which I was planning on doing anyway).
I think they have a pretty interesting case but I totally agree (as I mention) that it is very flimsy.
Indohyus and kin may really be no different than the hippos ...yes, propinquity in time and space matters but it is not definitive.
It is a good hypothesis, though. More than enough to argue for more funding for looking for more fossil in this time and place, and to suggest what to look for in the common ancestor.
You are missing two very important references: a morphological phylogeny from 2003 (in JVP) that finds whales and hippos as sister-groups, and another morphological one from 2005 (in PNAS) that confirms this finding and additionally finds the hippos to be anthracotheres, answering the question of where the hippo fossil record was hiding. In other words, the contentious issue in artiodactyl phylogeny is not the position of the whales anymore -- it's the position of the ruminants that differs between morphological and molecular trees! Indohyus does not shake anything; it resolves the position of the previously very poorly known raoellids in an unexpected place and has a few interesting implications for the process of whale origins. That's all. (Still deserves a Nature paper, though.)
I'll try to find your e-mail address and send you the pdfs.
Also, the skull with the cetacean middle ear has herbivorous-like molars in combination with cetacean premolars. (Large photo on Pharyngula.) It clearly belongs to Indohyus.
It's much less common than the reverse, but it has occasionally happened: turtles, marsupial lions... But the phylogeny of Eocene artiodactyls is not resolved in detail, and today's tragulids and stuff are omnivorous, as are, to some degree, the duikers; pigs are omnivorous anyway, so it could be that artiodactyls are ancestrally omnivorous.
David; Thank you for the papers, and I will read them as soon as I get the chance. I have no problem with cetaceans within the artiodactyla and hippos as a sister group, but there apparently still seems to be some debate about the issue because in the Indohyus paper the researchers made the Raoellids a sister group to Cetacea, both being a sister group to all other Artiodactyla. I don't agree with that arrangement, but I just wanted to point out that it's there. I still think some more resolution is needed on the hippo line as well, even though finding hippos to be anthracotheres is a big help.
Like I noted in the post, I don't think that Indohyus does what the Nature paper says it does, but I picked the title I did to draw attention to the hypothesis being proposed before going into it.
As for herbivory to carnivory (and things in-between), I think it's likely that the common ancestor for the raoellids was an omnivore. Unfortunately the soft-tissue material of the digestive tract will probably never show up in the fossil record of these animals, so teeth and isotopic data seem to be the best we can hope for. Still, there have been reversals of herbivory to carnivory at various times depending on how far back you want to go in the fossil record, and there are plenty of costs/phsiological barriers that go along with going from eating plants to eating meat. If anything I would think that the last common ancestor would be an omnivore rather than a strict herbivore, but then again Indohyus might not be a good model for cetaceans earlier than Pakicetus.
Talking about of my posterior here, but are there any good primitive hippo fossils? Is is possible that Indohyus and its raoellid cousins represent early hippos? Perhaps the cetacean middle ear is more widespread among that clade than we think, and that it was lost in hippos?
Again, the above is based on NO research at all. Just playing devil's advocate.
It's interesting how easily we assume the fossilized remnants of certain creatures are actually ancestors of others. If paleontologists could find a successive line of complete fossilized skeletons demonstrating evolutionary progression, such conclusions would be much more believable. The fact is that no such discovery has been made, but we are still assuming the principle is valid. I could come up with other alternative suggestions that are equally plausible, but are still just as unscientific as some of the hypothetical ideas presented by so many paleontologists. I'm just waiting to see some good hard data to validate what is theoretically predicted to have happened. Then maybe I might agree that evolution is an unassailable truth.
Douglas; Thank you for your comment, but I think that the version of evolution you claim to desire evidence for is stilled stuck in a bit of the "scala naturae" mode of thought. Old phylogenies used to try and show that the direct lines of descent for certain creatures based upon known fossil taxa, but current studies (based on cladistics) are more concerned with finding sister groups and determining relatedness to shed light on evolutionary relationships. Because evolution is a branching process in which populations evolve (and so one population of a species could be markedly different from another, as in the tool cultures of chimpanzees), it's impossible to line all the fossils up in a row and say "This fossil represents the direct ancestor of this other fossil" except in exceptional cases.
Further, we're lucky we have anything preserved at all as fossilization is a rare process. The study of whale evolution, as I noted in the post, has exploded during my lifetime, and there are a lot more known transitions now know than in 1983 when Pakicetus came to light. Whenever such a form is found it creates two more gaps so there will never be a successive, complete line leading from a terrestrial artiodactyl to a whale, but the lineage as it stands now is amazingly complete compared to others. Looking at other, less-charismatic, organisms like forams reveals an even more complete line (which is more likely to provide the line of descent to the clarity you're searching for).
You assail paleontologists, yet you don't seem to offer any specific example or address this post in detail. I don't agree with the taxonomic arrangement provided in the paper, but it's an interesting hypothesis that merits further study if nothing else. As for evolution itself, as I noted a few weeks ago it is a fact; life has changed dramatically over time, and this is supported by many intertwining lines of evidence from various disciplines. The mechanisms of evolution, however, (the "theories of evolution") are what are being debated, tested, checked, etc., so there is no reason to assume that Indohyus, Pakicetus, or Basilosaurus were created, intelligently designed, or anything else. Natural selection (along with kin selection, sexual selection, etc.) provides the best explanation for the main driving mechanism of evolution and I doubt it will ever be overturned (even if in the future it is diminished in importance for some reason I can't predict), and there is no reason for these researchers or any other serious scientist to start accepting some form of creationism because we don't yet have all the answers. Evolution as a science is still relatively young and has come a long very in a very short time, standing up amazingly well despite the protests of theologians and others since the beginning (and the arguments of modern day creationists are little different from those initially floated when Darwin's theory of evolution by natural selection came to light).
I assume that you are not a Neo-Lamarckian or hold a different view of evolution, so if you do not think evolution has taken place, from whence did these creatures spring?
It happened actually very often that herbivorous animals began to ate also other animals. I once wrote an article about carnivory in hooved mammals ( http://bestiarium.kryptozoologie.net/artikel/fleischgeluste-bei-huftier… ), and it is really amazing how many "normal" plant-eaters also consume meat. Especially hippos are well-known to eat carrion and sometimes even hunt other animals. Some of them like duikers actually hunt very often other animals. Chevrotains are especially interesting, as the are on the one hand extremely primitive, but on the other hand very specialized. They do not only consume plants, but also a wide variety of invertebrates and also small vertebrates. The most interesting fact is most probably that they do not only eat fruits which they found underwater, but also sometimes catch small fish. I can very well imagine that the earliest ancestors of whales actually had a similar way of life.
Sordes: Very interesting. I can't read your article in the original language, and the Babelfish "translation" is of limited help, so you may or may not already be aware of this:
In North America, whitetail deer have been known to eat birds as live prey (fledgelings, for example).
Squirrels and mice also eat baby birds I believe.
Douglas: NESTED HIERARCHY!
dense limb bones counter the buoyancy of skin fat and reduced fur in (sub) tropical (semi) aquatics, like hippos, manatees and coastal Homo erectus, so perhaps Indohyus was gradually losing its fur coat and adding some subcutaneous fat. I tend to see it as omnivorous though primarily herbivore, between rat and pig, rather than deer like. Interesting that hippo fossils don't go back so far, they eat grass, but according to this paper C4 grasses weren't so abundant before 30ma, so perhaps a diet change was involved. Both Indohyus and hippos lack the long neck of a browser.
link (just a copy, with my interpretation at top)
A well written and well researched article, but I fear that the general public may well assume that whales evolved from rodents - given the analogies used in the original paper. It only serves to demonstrate how fragmented our knowledge of the evolution of cetaceans really is.
:-o
I don't know how good the early anthracothere record is...
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Douglas, what do you expect? The "tree of life" is a bush, not a pole. It branches all the time. Today there are over 6200 species of amphibians alone, for example. Now consider how unlikely fossilization is (except for, say, equatorial Pacific diatoms). With our tiny sample, what is the probability that we ever find a direct ancestor of anything?
Besides, there's the science problem. Once we have an ancestor, we can't prove that it is one. If it lacks innovations of its own and has the right age, it enters consideration, but that's no proof; it might have had innovations that aren't preserved, for example.
As mentioned, Indohyus is not a direct ancestor that we happen to know. It has its own innovations, and it's no older than the earliest whales (which, incidentally, look very, very similar -- basically just their molars are different).
Oh, and... in science, there is no such thing as "unassailable truth". Science is incapable of proving. It can only disprove. As long as you can answer the question "if I were wrong, how would I know?", you are doing science.
DDeden, stop talking nonsense about Homo erectus. A bit of extra bone in the occiput is not going to help any; the head is already heavier than water. Thick-walled limb (!) bones are common in all amphibious tetrapods, they are not limited to tropical and/or subtropical ones (e. g. otters, beavers and muskrats have that, too) -- why should they be!
I'm in the lab of Michel Laurin, where research on this topic happens; several papers have already come out of it.
They don't seem to have evolved earlier than that.
Steve: Yes, I already wrote about the deer which eat young birds (sometimes they also only eat the limbs), also about the reindeers which hunt lemmings, the camels which eat carrion, the various things duikers hunt and eat, and some other things. Darren Naish sent me some time ago a very strange photo showing a giraffe chewing on a bone.
I just gave up and linked to you. Good article.
Every time I visit this blog I hear the opening bars to Layla. Sciblogging synaesthesia.
I've been trying to write an uber-post...y'know, to establish in my own mind that my blog is more than just random gibberish...and this was gonna be my topic.
DM: A bit of extra bone in the occiput is not going to help any; the head is already heavier than water. Thick-walled limb (!) bones are common in all amphibious tetrapods, they are not limited to tropical and/or subtropical ones (e. g. otters, beavers and muskrats have that, too) -- why should they be! I'm in the lab of Michel Laurin, where research on this topic happens; several papers have already come out of it.
They (C4 grasses) don't seem to have evolved earlier than that.
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I wasn't speaking of the occiput. I mentioned (sub) tropical because the Indohyus probably lived there, not in the arctic, which is why it probably lost the fur coat. Grasses may have been eaten by sauropods. Who/where is Michel Laurin? Paleo lab?
Some grasses may have lived long before, grass-type phytoliths in Indian Sauropod dung.
grass-dino link
Generally robust (heavy) skeleton, compared to any other primate or hominid, dense femur with narrowed cavity in Homo erectus.
Richard Leakey & Alan Walker: mentioned very heavy bones of Homo erectus at Turkana.
Marc Verhaegen: (Homo erectus) HE was the Homo fossil with the thickest cortical bone & narrowest bone marrow: skull, pelvis, leg bones, nearly all the skeleton. Probably the dense bones compensate for the fat. Fat tissues have a density of ca.0.9, but bones are much heavier than water (freshwater 1, sea water 1.024).
Sorry for the delay...
Oh man. You don't know who works on your favorite topic? Here's his homepage.
Yes, but not C4 ones.
Quantify, please.
(In general, you can't do science without math.)
Looks just like a water rat skeleton.