Evolution transcends down to every living organism on the face of the earth today from the last unicellular common ancestor (or LUCA). We have grown amazingly complex over the epochs. But, the essence of life or the one we know to say the least, remains the same. How organisms develop from a single cell to the myriad of cells in an adult shares a far greater mechanism across the taxa than the most ambitious of designers could have aimed for. But to say that looking at development is equivalent to looking at evolution, as Haeckel did in his famous hypothesis, ontogeny recapitulates phylogeny, would be an overstatement.
Evolutionary developmental biology or evo-devo is about looking at development processes of different species to deduce the ancestral relationship between them. Which factors, then, make the field more convincing than the recapitulation theory?
Modularity: Genetic networks are increasingly being looked at as modules. Different genes orchestrate together to achieve biochemical functions. To achieve the same function in different organisms, we would expect the concerned genetic module to remain intact across them. Modules are not restricted to the molecular level. Pentadactyly, for instance, is a phenotypic module.
Differential gene expression: A gene which may act as a switch in one organism may influence another gene in some other organism, leading to similar or different results. Surprisingly, the same gene may have different effects in the same organism at different times. The findings that large evolutionary changes occur due to differences in gene expression rather than evolution of new genes is proof that developmental changes precede evolutionary changes and not succeed them as originally expected.