The theory's thesis: "Ontogeny recapitulates phylogeny." Don't worry - it's not as complicated as the biological jargon might imply.
The idea boils down to a simple one - one that seemed to make sense in light of the fact that the science of developmental biology had only just begun from a systematic standpoint. The idea: if you watch an organism develop from an embryo to an adult, you can watch it slowly move through the evolutionary steps that had created it. That is: development repeats evolution.
So a human embryo would first look like a fish, then a reptile, then a mammal, and finally a human. Of course, we now know that in a literal sense, the theory is completely and utterly wrong. No stage of human development, or of any other organism, correlates with a discrete step in evolution.
We are never fish. (Though we do have embryonic tails).
However, that doesn't mean that there aren't kernels of truth to the idea, if applied loosely. Take the most famous and classic example: embryonic human gills. You may have heard yourself that humans have gills as embryos. Unfortunately this claim arises from misconception and incomplete understanding of developmental biology. Humans do not - ever - have gills. But as embryos we do have "pharyngeal arches." These are little bumps around what you might consider the neck area of a developing embryo (see Haeckel's drawings above). And these little mounds of tissue do in fact remarkably resemble similar mounds found in fish - mounds that in fish develop into gills (Note: Haeckel vastly oversimplified these drawings. I use them here as a simple illustration of the concept of developmental similarity. See: http://zygote.swarthmore.edu/evo5.html. Thanks Bjørn!).
One of the amazing aspects of developmental biology that much of the public does not generally understand is that evolution does not occur by adding new organs, appendages, or tissues to adult animals (whether through gradual steps or not). Evolution works by slowly sculpting the early embryonic clay of an organism.
Fish evolved these gill pouches as embryos - pouches that could then be sculpted into gills. As evolution waltzed and hopped along at its geological pace, genetic mutations began to change how these little mounds were sculpted, such that now in humans, these arches are sculpted into various parts of the face and head. A genetic program was already in place to control the shaping of the pouch. All that natural selection did was slightly tweak that program. For example, instead of a group of cells moving one direction, they moved another. Instead of becoming blood vessel cells, they became cartilage or bone cells.
Thus, while we now understand that we are not witnessing evolution in miniature during development, we are seeing pieces of our evolutionary history - little remnants that remind us of our relationships to our ancestors and also help inform us on what morphogenetic processes underlie evolution.
Which brings us to our adaptation of the week: the freaky asymmetric eyes of the flatfish.
Most people have probably seen a flounder - one member of the flatfishes. They have adapted to lie amongst the silty ocean bottom, hidden from predators and prey, flat on their sides. For a normal fish this might be maladaptive - they would constantly have one eye buried in the sand. Of course the negative of being one-eyed might be offset by being much more camouflaged and undetectable.
Luckily for the flounder, the eye that should be buried in the sand has moved around its forehead so that both eyes are on one side.
The flatfish eye served as one line of attack against natural selection back in the day - and Darwin himself didn't quite know how to answer the charges. Evolutionary gradualism would predict that through successive steps, the eye slowly moved upward toward the forehead and eventually to the other side of the face. But what advantage could a slightly moved eye give, if it still was on the wrong side? Alternatively, as Richard Goldschmidt postulated in the 1930s and 40s, perhaps a single monstrous freakfish was born with both eyes on one side, and this allowed it to lie flat without losing half its vision. It could have then survived and had lots of little freak fish babies of its own.
So how did the flatfish become the strange creature it is now? Let's first look at the developmental biology of the flatfish eye.
It's been know for quite some time that flatfish larvae look like perfectly normal, symmetrical, and upright fish. The picture to the right is from a study by Alexander Schreiber in the Journal of Experimental Biology from 2006. As you can see, at early stages the larvae are normal, but progressively tilt and become horizontal as one eye moves across the face. He also showed in this study that eye movement and flattening behavior occur independently during development - but that's a much longer story.
Alright, so one eye gradually moves across the skull during development. What about during evolution? Do we have any clues as to the steps involved? Well, as many biologists know, the fossil record has now answered the question for us.
In a well-known study that was published last summer in Nature and received much media attention, Matt Friedman showed findings from a series of fossils delineating a clear gradual evolution from symmetrical to asymmetrical flaltfishes. (For excellent in-depth coverage looking at this study and the debate over flatfish evolution, see one of my favorite science bloggers, Ed Yong at Not Exactly Rocket Science, and also see the popular science writer Carl Zimmer at The Loom, and GrrlScientist at Living the Scientific Life).
The evolution of the flatfish eye seems to mirror what we see during development. Thus, here we have a case of ontogeny appearing to recapitulate phylogeny quite wonderfully. There are many excellent examples of this throughout the biological world, though few that show such incredible similarity between the two processes of development and evolution. Nonetheless, this isn't really evolution we're watching during flatfish development - we're merely seeing how slight changes of the developmental programs are themselves responsible for the changes we see over time through evolution. Generally speaking, earlier developmental processes appear much more similar across varied species than later processes.
Development is in fact one of the primary constraints against evolution.
So while you were never a fish, you still showed remnants of fishy development during your own development. For it was these fishy developmental process that allowed the evolution of your own.
- Schreiber AM. Asymmetric craniofacial remodeling and lateralized behavior in larval flatfish. J Exp Biol. 2006 Feb;209(Pt 4):610-21.
- Friedman M. The evolutionary origin of flatfish asymmetry. Nature. 2008 Jul 10;454(7201):209-12.