18) The study of EDB in humans beings (by Rey J. Rosa Morales)

Figure 1. Human embryo stages.

         
        One of its most important and fascinating endeavors for evolutionary developmental biology will be to elucidate the developmental genetic and evolutionary mechanisms involved in the appearance of traits unique to humans, such as our large brain (figure 2) size, craniofacial morphology, vertebral, limb, and digit innovations, reduced hair cover, and, of course, our complex behavioral and cultural traits (figure 3 and 4). Based on studies in model organisms, we can wait that many of the innovations that evolved in the human lineage involved several or many genes. Comparative genomic data indicate that many or most of the DNA-level changes responsible caused alterations in the regulation of developmental and structural proteins that we share with our primate and mammalian relatives. A conservative estimate of the divergence in single-copy nucleotide sequence between the human and chimpanzee genomes is about 1.2 percent. Given the human genome size of about 3 x 109 base pairs, and assuming that half of this divergence (i.e., 0.6 percent) occurred in the human lineage, about 18 million base pair changes separate humans from the common ancestor we share with chimpanzees. According to Carroll (2003), assuming that the approximately 30,000 human protein-coding genes encode proteins with all average length of 400 amino acids, then only about 1.5 percent (270,000) of the 18 million substitutions will reside in protein-coding regions, and only about 200,000 of these will result in amino acid replacements, the rest being synonymous substitutions. This number could be an overestimate, as there is expected to be relatively less divergence in coding than in noncoding regions.

Figure 2. Human brain

Figure 3. (A) Urbilateria is the archetypal animal that was the last common ancestor shared by protostomes and deuterostomes. (B) The new animal phylogeny, showing that cnidarians are basal to bilateria and that protostomes are divided into two branches, the molting Ecdysozoans and the nonmolting Lophotrochozoans (De Robertis et al., 1990).

Figure 4. The Hox complex has been duplicated twice in mammalian genomes and comprises 39 genes. Note that microRNA genes, which inhibit translation of more anterior Hox mRNAs, have been conserved between Drosophila and humans (De Robertis, 2008).

        It will be an enormous work, much more complex than simply sequencing the human and chimpanzee genomes, to determine which of these amino acid replacements, and which of the many millions of potential regulatory DNA substitutions, have been involved in adaptive evolution of human morphological and behavioral traits. The answers to our questions about specific adaptations will have to come on a trait-by-trait and gene-by-gene basis analysis. This work will be of great interest not only to evolutionary biologists, but also to medical and pharmaceutical researchers. Many of the initial clues to the genetic bases of human traits come from studying human variation, including genetic disorders, and development in mammalian model species, such as the mouse. One critical and long-standing question that "human EDB" should eventually be able to shed light on is whether the genes underlying trait variation within the human species are the same as those involved in divergence from our relatives (Figure 5), the great apes. We can expect that such evolutionary knowledge will can apply concurrently and synergistically with advances in medicine and human developmental biology.

Figure 5. Human evolution 

References:

1. Carroll, S. B. 2003. Genetics and the making of Homo sapiens. Nature 422: 849-857. Chen, F.C., and W.H. Li. (2001). 
2. Genomic divergences between humans and other hominoids and the effective population size of the common ancestor of humans and chimpanzees. Am. J. Hum Genet. 68: 444-456.
3. De Robertis, E. M., G. Oliver and C.V.E. Wright (1990). Homeobox genes and the vertebra body plan. Scientific American 263, 46-52.
4. De Robertis, E. M. (2008). Evo-Devo: Variations on Ancestral Themes. Cell 132, 185-195.