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.
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.
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?