Re: relationship between development of human embryo to evolution

The statement "ontogeny recapitulates phylogeny" (sometime called the Biogenic Law) refers to the erroneous idea that all evolutionary change involves the addition of new features to the end of the developmental process. In this view, the developmental stages of an organism represent all of the adult stages its predecessors passed through during their evolutionary journey. The person credited with this view, the German anatomist Ernst Haeckel, was so taken by it that he manipulated his descriptions of the embryonic stages of various organisms to better fit his preconceptions ( see Richardson et al 1997
That ideas can blind is a human failing, certainly not unique to scientists.

In part the idea of the "Biogenic Law" was based on the view that biological evolution represents a progression toward increasing complexity (e.g. humans as the crown of creation), together with the view that events early in an organism's embryonic development are difficult if not impossible to alter through natural selection. Modern studies of evolutionary change and developmental biology indicate that both views are fundamentally flawed (see Richardson, 1999)

The Darwinian view is that evolutionary history is driven primarily by adaptation to specific environmental niches (lifestyles). However, whether such specializations involves increasing or decreasing complexity (i.e. simplification) depends upon circumstance. Many parasites have been able to become highly successful, from an evolutionary perspective, by deleting extraneous features and "simplifying" themselves; they are lean and mean. This processes of specialization by simplification occurs repeatedly in the history of life.

As to the second point, while it is clearly the case that organisms share many common developmental processes (indicating their common evolutionary origin), it is also quite clear that all stages of embryonic/ organismic development can be targets for natural selection. Moreover, the development of an organism is not a monolithic process, but involves a number of parallel processes and events, many of which can vary independently of the others. The development of a particular organ/tissue can be accelerated or delayed with respect to others. There are many examples among many different groups of living organisms in which two related species have very different early developmental processes, differences that appear to be different primarily because of natural selection acting on the embryonic stage. Consider for example the frogs, all of which share a rather similar structure as adults. There are the "typical" frogs have develop in an aquatic environment and pass through a feeding tadpole stage and then metamorphose into adults, whereas others (which live completely on land) have deleted the larval stage and hatch as adults. The early developmental stages of these organisms can appear quite different, yet produce similar adults (i.e. frogs). An accessible discussion of these processes can be found in Kenneth McNamara's book, Shapes of Time: The evolution of growth and development. (1997). Johns Hopkins Press.

So the end result is that selection can act at many stages during the life of an organism, and a species can grow simpler or more complex over time. Since new species can arise for species that have previously undergone "simplification" the tree of life has a rather convoluted branching pattern (rather than than the overly simple one implied by the incorrect phrase "ontogeny recapitulates phylogeny").