The evolutionary geometry of human anatomy: Discovering our inner fly

Abstract

The human body is one still frame in a very long evolutionary movie. Anthropologists focus on the last few scenes, whereas geneticists try to trace the screenplay back as far as possible. Despite their divergent time scales (millions versus billions of years), both disciplines share a reliance on a third field of study whose scope spans only a matter of days to months, depending on the organism. Embryology is crucial for understanding both the pliability of anatomy and the modularity of gene circuitry. The relevance of human embryology to anthropology is obvious. What is not so obvious is the notion that equally useful clues about human anatomy can be gleaned by studying the development of the fruit fly, an animal as different from us structurally as it is distant from us evolutionarily. The underlying kinship between ourselves and flies has only become apparent recently, thanks to revelations from the nascent field of evolutionary developmental biology, or evo-devo. All bilaterally symmetric animals, it turns out, share a common matrix of body axes, a common lexicon of intercellular signals, and a common arsenal of genetic gadgetry that evolution has tweaked in different ways in different lineages to produce a dazzling spectrum of shapes and patterns. Anthropologists can exploit this deep commonality to search our genome more profitably for the mutations that steered us so far astray from our fellow apes.

Genetic and developmental basis for parallel evolution and its significance for hominoid evolution

“…This has resulted in the startling discovery that even in greatly diverged groups the repeated evolution of certain structures occurred through the reuse of homologous genes . Classic examples include the eyes ( eyeless/Pax6 ), heart ( tinman/Nk2 ), and limbs ( distal‐less/Dlx5/6 ) in animals as diverse as flies and vertebrates . This paradox is explicable by the realization that deep conservation of the gene regulatory networks involved in animal development actually facilitates the repeated evolution of similar phenotypes.…”

“…7 Classic examples include the eyes (eyeless/Pax6), heart (tinman/Nk2), and limbs (distal-less/ Dlx5/6) in animals as diverse as flies and vertebrates. 6,9 This paradox is explicable by the realization that deep conservation of the gene regulatory networks involved in animal development actually facilitates the repeated evolution of similar phenotypes. Such examples are not limited to repeated evolution in distantly related species in which homoplasy is readily apparent.…”

Genes, development, and evolvability in primate evolution

“…For example, the homeobox gene Antennapedia determines which body segments along the fly's anterior‐posterior axis will form legs. A mutation that results in the loss of function of this gene causes these body segments to grow antennae instead of legs . Structurally and functionally similar genes perform similar tasks in patterning vertebrate embryos, including human ones .…”

“…These vertebrate homeotic, or Hox, genes are recruited many times and in multiple tissues during vertebrate morphogenesis and development (Fig. ), giving rise to a central concept in evolutionary developmental biology: the developmental genetic tool kit . Developmental tool kits are subsets of genes used to specify animal form during embryological development.…”

The red queen and her king: Cooperation at all levels of life

“…A broader view will also make sense of evolutionary questions, such as those pertaining to the nature—sometimes quirky—of human morphological traits that might otherwise be difficult to explain (e.g., Held 2009; Held 2011). This framework is also important in formulating study designs and evaluating results to provide improved explanations of human evolution and behavior that are of such interest to anthropologists.…”