This concept was popularized by scientist Ernst Haeckel (1834-1919), who suggested that humans, and other animals, would resemble different stages of their evolutionary history during development. Humans, for example, might show stages where they look like a fish, amphibian and monkey before settling on their unmistakable human form. It turns out that some of Haeckel’s drawings in support of this hypothesis were likely doctored, a point that has been harped on by creationists [1, 2] as proof of the dishonesty of at least some proponents of evolution.
While some of the details of this hypothesis are a bit extreme and not accepted by evolutionary biologists today, we do think that development holds important clues about evolution. Indeed, there is a growing amount of evidence that changes in the DNA that regulates when certain genes are turned on and off during development may lead to the most striking evolutionary changes in anatomy.
The cases that provide the most obvious evidence of evolution involve instances where the genes for developing a structure are apparently turned off, but not right away. Such instances give us fleeting glimpses of traits that were likely inherited from an ancestor, but new adaptations have led such traits to degenerate or change at later developmental stages.
Perhaps the most well-known example of this involves tails in humans. Humans do not have tails, of course, but most other mammals and, importantly, most Primates do. If we were to share a common ancestor with these tailed species, the simplest explanation is that we descended from ancestors that had tails. Unfortunately there are no ‘tail genes’ that we know of, so we cannot look for remnants of those, but it turns out that embryos provide a different sort of remnant.
As you can see in the above developmental series, humans initially have a tail. The tail eventually undergoes programmed cell death (apoptosis) and is reabsorbed, but its implications are unmistakable: It is fully consistent with the idea that humans descended from tailed ancestors.
Throughout this blog, I will be highlighting similar examples of how development can shed light on our evolutionary past, and when paired with data from DNA and fossils, these data often tell the same story: that life has evolved.