A comparison of vertebrate embryos at the same stage of development. (Courtesy: G. Romanes)
"It is notorious that the wings of birds and bats, and the legs of horses and other quadrupeds, are undistinguishable at an early embryonic period, and that they become differentiated by insensibly fine steps." Thus writes Darwin (6th ed., p. 226), using this observation regarding individual development of organisms in defense of gradual evolution along ancestral lines.
Individual development of an organism is called ontogeny. Its line of ancestors defines its phylogeny. Is there a connection between the two? Does ontogeny ndeed provide clues for ancestry, as claimed by Darwin?
The most successful group of organisms on this planet, when species diversity is used as criterion is the insects. They have invaded every nook and cranny in the terrestrial realm, including the freshwater bodies. We might think that beetles are abundant, but their larvae "grubs" are even more abundant, except that they are hidden in the ground. Many other multicellular organisms have immature stages that look very different from mature ones. Besides the larvae of insects, the tadpoles of frogs are perhaps the best known examples, to us land dwellers.
In the ocean, the larvae of nearshore organisms populate the coastal ocean as microscopic "meroplankton". The meroplankton has larvae from corals, sea stars, polychaete worms, barnacles and crabs, mussels and snails, and many others. It is from such larvae (which look nothing like the adult stages) that one can tell quite readily that barnacles and crabs belong to the same general tribe of animals. The same is true for mussels and snails.
In terms of function, the large differences between larval stage and mature animal are readily appreciated. There are two strategies involved, addressing two different needs. One is the search for resources and the other is dispersion. In insects with a distinct larval development, the larvae are responsible for feeding and putting on weight, while the main task for the winged stages is to seek mates and look after dispersal. (Additional food input is used for maintenance and to make more eggs.) In the coastal ocean, the adult sessile (permanently attached) stages are responsible for growth and reproduction, while the main function of the larvae is dispersal, that is, finding good places to settle. (Of course, the larvae also feed, to maintain their metabolism and to get a good head start when settling.) In frogs, both larvae and mature animals feed and grow, but they exploit different habitats, one within the water, the other at its edge or outside of it.
The fact that the larvae of many distantly related organisms resemble each other more closely than do the mature animals (e.g., barnacles and crabs; tunicates, tadpoles and fish), in addition to observations regarding vertebrate embryos (quoted above) drew attention to the possibility that there are clues, within development, to relatedness and hence evolution. This idea, fundamentally sound, gave rise to the catchy phrase "ontogeny recapitulates phylogeny" which implies that development repeats aspects of the stages of evolution within a single organism. When taken literally this is nonsense; neither human embryos nor those of poultry have a fish stage. However, when taken as an invitation to look for clues of the course of evolution it does make sense. Human embryos, chick embryos and fish embryos have much in common in their early developmental stages as was seen in another section.
The German biologist Ernst Haeckel (1834-1919), more than anyone else, insisted on the importance of ontogeny as a clue to the "Tree Of Life". This is his best-known contribution to the life sciences. (He also was the world's expert on radiolarians, calcareous sponges and coelenterates in his time. A powerful writer and lecturer, he urged the theory of evolution on his colleagues and the public.) In his view, the reason that ontogeny proceeds the way it does is that each organism remembers its evolution. Early development is driven by ancient programs; the earlier the development, the more ancient the program.
Whether this is true or not for any given organism cannot be decided by disputation. The answer will come from the study of the genetic code (a concept not available at Haeckel's time). Inasmuch as evolutionary pressures act on larvae, as well as on adults (larvae have to find food and escape predation), we should expect to find modifications in the genetic code to improve adaptations of larvae. Embryos have other problems: they must convince their parent that they need more resources. The corresponding signals presumably would be different for sharks, birds and mammals, leading to a divergence of developmental programs. Because of these caveats, the concept that "ontogeny recapitulates phylogeny" cannot stand as a general rule.
In any case, the patterns of development in animals, plants and fungi have been widely used to generate classifications that are thought to reflect relationship in terms of common ancestry. Clearly, whenever two different organisms have complex but similar developmental programs, it must be assumed that they inherited their programs from a common ancestor, rather than that the programs arose independently, by chance.