Fetal whales have hindlimb buds

DNA suggests that whales are descended from hoofed mammals, and the fossil record also appears to document a transition of four-limbed whales to modern species that only have forelimbs. Looking at the ontogeny, or development, of whales also provides evidence of a past when the ancestors of these aquatic creatures walked on land.

Below is a picture of an Atlantic spotted dolphin (Stenella frontalis), which very clearly has forelimbs (flippers) but no hindlimbs, typical of whales.

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But when this and related species are still embryos, you can see that tiny hindlimb buds form [1].

spotted-dolphin_embryo

If you look at a series of embryos through their development, the hindlimb buds form but then they disappear.

dec05f3

Interestingly, dolphins seem to retain some of the genetic developmental machinery to make hindlimbs. Occasionally, people have found individual dolphins that have hind flippers, such as the one in the picture below.

4331254_orig

This is called an atavism, and is thought to arise from one or more mutations that somehow turn the development of the hindlimbs back on.

Together, this is a remarkable example where DNA, fossils and ontogeny all tell the same story, specifically that whales descended from ancestors that walked on four legs. These animals appear to have gradually lost their hindlimbs,  presumably as an adaptation to streamline their bodies for swimming, but their ontogeny seems to retain a record of their fully-limbed past.

Questions for Creationists

Is it just a coincidence that DNA, fossils and ontogeny all suggest that whales descended from four-limbed ancestors? Why would God create dolphins with hindlimb buds that simply disappear? Is it just a coincidence that we see the same pattern of limb buds forming and then disappearing in legless lizards?

References

1. Thewissen, J. G. M., Cohn, M. J., Stevens, L. S., Bajpai, S., Heyning, J., & Horton, W. E. (2006). Developmental basis for hind-limb loss in dolphins and origin of the cetacean bodyplan. Proceedings of the National Academy of Sciences103(22), 8414-8418.

 

Photo credit

Adult spotted dolphin, dolphin embryo, embryo series, dolphin atavism

3 thoughts on “Fetal whales have hindlimb buds

  1. Ryan S.

    These are just lumps. How do you know they are limbs? Do they have bone, muscle, nervous system? How do you know the formation of these “vestigial limbs” are related to former land mammals? – the photo of a dolphin with rear fins rather shoots your argument in both feet because this appears more likely to be unneeded fins that are disappearing, rather than unneeded feet. Where is the study of the genome of the dolphin that shows its “limb buds are due to vestigial feet of an earlier land mammal and how does the developing fetus deal with these limb buds it doesn’t need? Given that the dolphin lost its rear legs early on in its evolution, why does that part of the fetus body appear prone to ativism and not some more recently evolved part of its body?

    Then we have the issue of “disappearing”. Disappearing where? Did the cells atrophy, die and get consumed by the growing fetus? Or did the fetus simply grow bigger while the “limb buds” stayed the same size? How do cells know how and where they need to be anyway? How do cells that need to be a liver decide “OK, I’m in the right place to be a liver, and not a brain – so I shall be a liver”, because this seems to be key to understanding what those “limb buds” are and where they go.

    The whale and dolphin developed from a land based mammal similar to a antelope around 50million years ago and the process of evolution took 15million years. The time period between generations for these large and complex animals is about 30 years, so the whale developed from an antelope in about 500,000 generations. The genome for a whales has about 2.billion base-pairs – so slightly less complex than a human being. Here are some differences between whales and antelope:-

    fins
    no rear legs
    no front legs
    lots of fatty tissue
    articulated rib-cage
    blow hole in the head
    eats meat not plants
    hidden penis
    no fur
    can withstand huge pressures
    skin impervious to water
    longer spine
    moves my manipulating spine
    baleen
    camouflage suited to a sea creature
    immense size
    sonar
    enhanced sense of hearing in water
    enhanced eyesight in water
    able to communicate in water
    highly intelligent with a relatively large brain mass
    sleep with one side of the brain at a time to avoid drowning
    can open mouths underwater without choking

    So that’s 23 differences I can come up with without thinking too hard. Even if I was generous and said each of those changes required just one base-pair change, I’d struggle to see how you could have that many changes in the whale genome of 2.3billion base-pairs without having huge numbers of species of “demi-whales” which don’t exist in the fossil record. In any case, that just isn’t possible as the cell-level error-correction mechanisms in the splicing of large complex genomes such as those found in humans and whales mitigate against rapid rates of mutation given that in long complex genomes, mutation likely results in a non-viable fetus or sterility.

    The usual response from Darwinists is “natural selection” but natural selection only subtracts information from the gene pool. The only posited method for adding new information to the genome is by cosmic radiation causing entirely random changes to the genome.

    So, the point I’m making here is “Why bother obsessing over “limb buds” that might not be in any way related to legs when the real question is: How did the antelope transform into a whale in just 500,000 generations of entirely random change to its own enormous genome?”

    By the way, Creationists argue that evolution, together with the big-bang theory is all just a fraud intended to mislead the unwary. Since this is an internally consistent there’s precious little point in arguing with them at all. So I don’t know who you are arguing with, but it isn’t Creationists. Right now you are discussing this with an engineer, so try and leave your religious beliefs at the door.

    1. Hi Ryan, thank you for your comment! First off, I’m sensing a bit of hostility from you based on your last sentence, so please play nice. No religious beliefs will be used to discuss this, as I am a scientist discussing science here.

      Regardless, there’s a lot to unpack here but I’ll do my best:

      “These are just lumps. How do you know they are limbs? Do they have bone, muscle, nervous system?”

      Yes indeed these are lumps, but these lumps are in exactly the place you find limb buds forming in mammals that develop limbs. See this human developmental series for comparison: https://embryology.med.unsw.edu.au/embryology/index.php/Embryonic_Development

      I’m not an embryologist, but if I had to wager a guess, bone, muscle, and nervous tissue are probably either (a) not developed or (b) poorly developed at this stage. If you’re interested, I can send you the primary literature for this, and you can see if they mention anything on it (I don’t recall anyone do tissue thin sections on these, but could be misremembering).

      “How do you know the formation of these “vestigial limbs” are related to former land mammals? – the photo of a dolphin with rear fins rather shoots your argument in both feet because this appears more likely to be unneeded fins that are disappearing, rather than unneeded feet.”

      By themselves, you are correct. It would seem more reasonable to think of them as regressed fins rather than regressed limbs used on land. However, when paired with evidence from DNA (https://evolutionforskeptics.wordpress.com/2014/07/20/molecular-phylogenetics-whales-are-hoofed-mammals/) and the fossil record (https://evolutionforskeptics.wordpress.com/2014/05/18/transitional-fossils-3-through-11-from-land-mammals-to-the-ancestor-of-whales/) suggests that these limbs are remnants of hindlimbs that ultimately would have been used on land. Or perhaps it’s better to think of it like this: we believe they were load bearing limbs, they were reduced in size and maybe were used as fins, then they were lost altogether.

      “Where is the study of the genome of the dolphin that shows its “limb buds are due to vestigial feet of an earlier land mammal and how does the developing fetus deal with these limb buds it doesn’t need?”

      Regarding the first part, there was a recent study that addressed this: https://academic.oup.com/nar/article/46/18/9299/5078245

      Regarding the second part, not sure what it has to ‘deal’ with as the buds simply appear to be broken down reabsorbed into the body, just like the tail in embryonic humans.

      “Given that the dolphin lost its rear legs early on in its evolution, why does that part of the fetus body appear prone to ativism and not some more recently evolved part of its body?”

      All of the evidence suggests that whales are notoriously slow evolving species, due to their low birth rate and high age at sexual maturity. As such, it’s conceivable that even if a trait is lost, there may not have been a ton of mutational change to completely erase all semblance of the formerly functional genetic elements, leading to the possibility of a reverse mutation to ‘reactivate’ some lost gene or gene modifier.

      I’m unsure how many atavisms have been recorded in whales and dolphins, but given that they’re sort of one offs, people probably generally don’t notice them. Hindlimbs really stand out, but an atavism in the pancreas might be less noticeable. The only other potential example that I’m aware of is the pineal gland. Whales and dolphins generally appear to lack a pineal gland, and we have evidence that it was lost before the last common ancestor of living whales and dolphins, but now and then you find one popping up. This is probably more noticeable because people are interested in dolphin brains.

      “Then we have the issue of “disappearing”. Disappearing where? Did the cells atrophy, die and get consumed by the growing fetus? Or did the fetus simply grow bigger while the “limb buds” stayed the same size?”

      Regarding the first part, it’s a process known as apoptosis or programmed cell death. There are a lot of developmental processes in which our bodies forms cells only to destroy and consume them later. The human tail is one I mentioned previously, but it happens beyond that. So the buds in the dolphin embryos form and then are destroyed and consumed (i.e., recycled) by the embryo.

      “How do cells know how and where they need to be anyway? How do cells that need to be a liver decide “OK, I’m in the right place to be a liver, and not a brain – so I shall be a liver”, because this seems to be key to understanding what those “limb buds” are and where they go”

      This is a really complex issue and one that I am not particularly well-versed on, but I’ll try: the cells in the developing embryo act upon each other and communicate with one another to determine their fate. The position of the cells is of major importance, such that cells closer to the outside of the embryo are going to be involved with becoming things like skin and nervous cells whereas more internal cells will become things like intestinal cells. There are many stages that involve folding, enveloping, migration, and chemicals are produced by the cells that cause other cells to do one thing or another. We know that there are special regions of DNA that help determine which tissues a gene is turned on in, and so the timing of when they’re activated matters a great deal. You’d probably be better served watching a video on this.

      “The whale and dolphin developed from a land based mammal similar to a antelope around 50million years ago and the process of evolution took 15million years. The time period between generations for these large and complex animals is about 30 years, so the whale developed from an antelope in about 500,000 generations.”

      There may have been a more robust analysis recently, but the sort of gold standard estimates for the past ten years puts the whale split from hippos at roughly 24 million years.

      Generation times vary among whales. A dataset known as PanTheria lists the average age at sexual maturity as 8.4 years, with the youngest being 2.7 years (I’m a bit skeptical about the latter, for the record). Plus, we don’t know what the generation times were like for the proto-whales. The very earliest putative whales (land based and/or amphibious) were rather small, and smaller mammals tend to have much faster generation times. The slowest generation times are going to be found in the most recent massive whales like blues and grays.

      Plus you’re assuming a single individual lineage giving birth in succession during those 15 million years. But it wasn’t an individual but populations. How big these populations were is of course difficult if not impossible to get a reasonable estimate for, but you’re going to have a lot more mutations and therefore genetic variation to work with in shaping these lineages.

      “I’d struggle to see how you could have that many changes in the whale genome of 2.3billion base-pairs without having huge numbers of species of “demi-whales” which don’t exist in the fossil record.”

      But we do have evidence of such “demi-whales”, as I briefly recount in the fossil blog post shared above.\

      “In any case, that just isn’t possible as the cell-level error-correction mechanisms in the splicing of large complex genomes such as those found in humans and whales mitigate against rapid rates of mutation given that in long complex genomes, mutation likely results in a non-viable fetus or sterility.”

      Mutations still happen though and they get through.

      “The usual response from Darwinists is “natural selection” but natural selection only subtracts information from the gene pool. The only posited method for adding new information to the genome is by cosmic radiation causing entirely random changes to the genome.”

      It’s not just cosmic radiation that induces mutations, but rather there are several mechanisms. Also the changes are not entirely random. Plus, the gene pool can also be expanded through hybridization leading to a process known as introgression, where another population’s gene variants can enter into one’s gene pool.

      “So, the point I’m making here is “Why bother obsessing over “limb buds” that might not be in any way related to legs when the real question is: How did the antelope transform into a whale in just 500,000 generations of entirely random change to its own enormous genome?””

      First off, the buds seem most likely to be associated with legs, but in regards to your question, we’re working on it! We have a lot of cool examples, some of which are highlighted in this blog, and many others that are being uncovered by groups around the world (since people like whales so much, lots of efforts in sequencing their genomes). I focus primarily on gene losses, but nonetheless there are a lot of convincing examples of whales having been land based species based solely on these:

      https://evolutionforskeptics.wordpress.com/2018/06/11/chitinase-gene-remnants-point-to-insect-eating-ancestry-for-rhinos-tigers-and-humans/

      https://evolutionforskeptics.wordpress.com/2014/10/05/pseudogenes-whales-have-remnants-of-tooth-genes/

      https://evolutionforskeptics.wordpress.com/2014/09/17/pseudogenes-whales-can-only-taste-salt/

      https://evolutionforskeptics.wordpress.com/2014/09/10/pseudogenes-whales-have-lost-their-vomeronasal-organ-and-associated-genes/

      https://evolutionforskeptics.wordpress.com/2014/08/25/pseudogenes-whales-have-lost-the-ability-to-discern-colors/

      “By the way, Creationists argue that evolution, together with the big-bang theory is all just a fraud intended to mislead the unwary. Since this is an internally consistent there’s precious little point in arguing with them at all. So I don’t know who you are arguing with, but it isn’t Creationists.”

      I have no interest in arguing with creationists. Arguing is a waste of time, given that people that are arguing probably aren’t honestly evaluating data and considering alternative explanations (i.e., seeking the truth). This blog is for skeptics who are curious, and many of those skeptics are creationists or former creationists who are trying to figure out what they believe.

  2. Pingback: When whales walked: Fossils document the rise of the largest animals on earth – Evolution For Skeptics

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