Ontogeny recapitulates phylogeny: 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.

14811288718_4fe28a376c_b

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

Ontogeny recapitulates phylogeny: Fetal sloth teeth point to evolutionary history

b3cc2ea20bb8f5f0801d88fed3d51dc3

Sloths are strange creatures in many ways. In addition to their slow movements and suspensory way of life, they have a chambered stomach like a cow, algae species that are found on their hair and nowhere else in the world, moths whose life cycles are entirely tied to sloths (“sloth moths”), surprisingly degenerate vision, and strange teeth.

Their teeth are bizarre in a number of ways: they lack enamel, they don’t have baby teeth (i.e., they are born with the only set of teeth they will ever have), and their teeth are notoriously difficult to categorize. By this I mean that scientists aren’t really sure if sloths have canine teeth or premolars. What does seem clear is that they do not have incisors, the front teeth typically present in mammals.

Based on evolutionary theory, scientists think that sloths descended from mammals that had all of the typical mammalian tooth traits, leading to the prediction that they may retain some features suggestive of this earlier dental history. For example, despite lacking enamel, sloths retain defunct remnants of enamel genes in their genomes. The ontogeny, or development, of sloths might give more hints of their toothier past.

Lionel Hautier and his colleagues [1] set out to examine sloth fetuses using CT scans to test this idea. After examining 25 sloth specimens, they found that sloths had very tiny teeth that are not present in adults. At least one set of teeth in the two-toed sloths (Choloepus) are replaced, suggesting they do indeed have baby teeth, but they do not remain once the sloths are born.

srep27763-f3

dCf stands for deciduous, or baby, teeth. Note that the two-toed sloth fetuses (A, B) have baby teeth (dCf) right next to the adult teeth (Cf) before the former disappear entirely when they’re born. C is a three-toed sloth for comparison.

Additionally, the researchers discovered that some of the tiny teeth in the three-toed sloths are incisors, which characteristically develop on the premaxilla of the upper jaw (red coloring in the images above and below). However, these appear to be resorbed by the time of birth. Interestingly, not all of the sloths develop with these tiny incisors, suggesting that they may currently be evolving such that they will eventually be completely lost.

srep27763-f1

Three-toed sloth fetuses, some of which (C,D) have incisors (dVpmx) that are not retained in the adults, as well as other teeth that disappear before birth (dv).

Questions for Creationists

Why do sloths have teeth that begin to develop and then disappear altogether? Would it not have been simpler for God to create sloths with only one set of teeth and not teeth that no longer function? Is it just a coincidence that sloth fetuses replace their baby teeth with adult teeth and have incisors, just like early fossil mammals and most mammals today? Why do we also see a similar pattern in baleen whales, which lack teeth entirely as adults?

References

1. Hautier, L., Rodrigues, H. G., Billet, G., & Asher, R. J. (2016). The hidden teeth of sloths: evolutionary vestiges and the development of a simplified dentition. Scientific reports6.

Photo credit

Three-toed sloth, deciduous teeth, incisors

 

Ontogeny recapitulates phylogeny: Legless lizard embryos have hind limb buds

As I discussed in a previous post, molecular phylogenetics suggests that many lizards evolved to become legless over time. One might wonder if there is any evidence of this legged ancestry during their development. In fact, at least some legless lizards possess limb buds as embryos. One particular example comes from the sheltopusik (Ophisaurus adopus), a legless lizard that is more closely-related to alligator lizards than snakes.

thumb3_xgQV2v

Below are six Ophisaurus embryos at different stages in development [1]. Despite being legless as an adult, you’ll notice that near the tail region there are tiny hindlimb buds (labelled “b”).

Ophisaurus embryos-page-001

For comparison, here is an embryo of a fully-legged anole lizard (Anolis).

longiembryo1

During development, the sheltopusik’s hindlimb buds fail to develop fully, though they’re retained as tiny remnants in the adults (they are above the more prominent hemipenes in the image below).

hbr884_1052r

In this way, the development (ontogeny) of the sheltopusik recapitulates its evolutionary history (phylogeny), with its embryos showing a glimpse of its legged past.

Questions for Creationists

Why do you think God would have created legless lizards that have limb buds during development? Is it a coincidence that they are genetically nested within fully-legged lizards? What other evidence might be used to test the hypothesis that legless lizards evolved from legged ancestors?

References

1. Rahmanl, T. M. Z. (1974). Morphogenesis of the rudimentary hind-limb of the Glass Snake (Ophisaurus apodus Pallas). Journal of embryology and experimental morphology32(2), 431-443.

Photo credit

Ophisaurus adultAnolis embryo, Ophisaurus leg remnants by Hendrik Bringsoe

Ontogeny recapitulates phylogeny: Baby baleen whales have teeth (before they lose them)

As a final note on my series on whales, I’ll mention an interesting tidbit about teeth. Despite the fact that baleen whales lack teeth entirely, fetal baleen whales do have tooth buds that never fully develop.

demere-tooth-buds_2_2

This ties in nicely with both the fossil and genetic evidence that baleen whales descended from species that lost their teeth as they transitioned to filter feeding.

Questions for creationists

Why would God create baleen whales with tooth buds that fail to fully develop? Is it a coincidence that whales have tooth pseudogenes and putative transitional fossils showing evidence of losing teeth?

Ontogeny recapitulates phylogeny: Why developmental biology matters

I still recall taking a class during my first year in community college where I was introduced to the phrase “ontogeny recapitulates phylogeny”. I asked the instructor what that meant, and he replied, “Well, think of the meaning of each word and put it together”. The truth was that I didn’t know what any of the words meant!

So here’s the breakdown: “ontogeny” = development of an organism; “recapitulates” = summarizes or recaps; “phylogeny” = evolutionary history of an organism. Put together: the development of an organism recaps it’s evolutionary history.

This concept was introduced by Ernst Haeckel (1834-1919) and he suggested that an organism, such as a human, would resemble different stages of it’s evolutionary history during development, e.g., at one point it would look like a fish, then an amphibian, then a lizard-like creature, followed by a generalized mammal, monkey and ultimately human. While this particular manifestation of the hypothesis is a bit extreme in its predictions, there is a good deal of evidence that some ontogeny does in fact recapitulate phylogeny consistent with evolutionary theory.

In fact, evolutionary biologists are becoming increasingly aware that major changes in anatomy through geological time may in fact be due to changes in gene expression patterns throughout development rather than simple mutations that directly confer a new characteristic. That is, rather than a simple mutation in a hypothetical tail gene causing apes to suddenly not have tails, a mutation in an underlying gene regulator may cause tails to develop initially but then undergo degeneration before development is complete.

Just this monday, I taught my classes that humans show evidence of this phenomenon. Humans are thought to be descended from animals that have tails, a feature that is not only common to many land vertebrates and mammals, but is specifically widespread in Primates, e.g., monkeys and lemurs. Humans are anatomically and genetically very similar to monkeys and thought to be derived from a monkey lineage in the Old World, so we are thought to have ancestrally possessed a tail. Sure enough, during development we do have a tail but this disappears via cell death by the time we’re born.

600px-Human_Carnegie_stage_10-23

You can see that many of the stages in early human development include a tail. If development doesn’t proceed normally, humans can actually be born with tails.

1883d6jlb7tb0jpg

Evidence such as this suggests that developmental biology can indeed reflect our evolutionary past, a theme that I will continually address in this blog.

Questions for Creationists

If evolution is true, do you think developmental biology can show evidence of evolutionary history? What kind of evidence would you predict to support it? If creationism is true, what kind of data from developmental biology would support it? Why would God create humans to have a tail during development but then have it disappear under normal circumstances?