Development and the evolvability of human limbs

Abstract

The long legs and short arms of humans are distinctive for a primate, the result of selection acting in opposite directions on each limb at different points in our evolutionary history. This mosaic pattern challenges our understanding of the relationship of development and evolvability because limbs are serially homologous and genetic correlations should act as a significant constraint on their independent evolution. Here we test a developmental model of limb covariation in anthropoid primates and demonstrate that both humans and apes exhibit significantly reduced integration between limbs when compared to quadrupedal monkeys. This result indicates that fossil hominins likely escaped constraints on independent limb variation via reductions to genetic pleiotropy in an ape-like last common ancestor (LCA). This critical change in integration among hominoids, which is reflected in macroevolutionary differences in the disparity between limb lengths, facilitated selection for modern human limb proportions and demonstrates how development helps shape evolutionary change.bipedalism | hominin | integration | macroevolution | serial homology

Allometric Variation in Modern Humans and the Relationship Between Body Proportions and Elite Athletic Success

“…Modern human limb proportions and stature evolved by 1.5 million years ago in Homo erectus (Ruff , 2002;Walker & Leakey, 1993) along with the ability to walk bipedally as effi ciently as modern people (Lordkipanidze et al, 2007). One of the phenotypic correlates of bipedalism is the decoupling of fore -and hindlimb proportions, such that arm span in humans is more independent from leg length than in other nonape primates (Young et al, 2010). Th e archaeological record indicates that human height was fairly stable from 100,000 to 50,000 years ago and has since experienced a small decrease in average stature (Formicola & Giannecchini, 1999;Ruff et al, 2002).…”

Linking a Biological Mechanism to Evolvability

“…Young et al [28], as noted by Brown [20], attributes differences in limb diversity between apes and quadrupedal monkeys to the release of an ambiguous developmental constraint. Brown grouped such developmental processes as a causal subset of X, noting the existence of other parameters, inherent to the relevant population, capable of causally influencing the probability of change to a future state.…”

“…Brown grouped such developmental processes as a causal subset of X, noting the existence of other parameters, inherent to the relevant population, capable of causally influencing the probability of change to a future state. Unique to Brown [20] and Young et al [28], however, the mechanism of PGC determination directly influences the modification of lineages in the context of descent. In other words, the mechanism of PGC determination is an 'X, ' a variable of utmost relevance that characterizes a piece of the inherent traits of a population and drastically influences its evolvability.…”

“…Teleosts and anurans, for example, are not bound by a character state [e.g. unequal limb length from Young et al [28] that separates them from respective sister lineages, but are rather constrained (or released) to a suite of phenotypes by other suites of constraints (or releases) less restrictive than the induced PGC determination mechanism. PGC determination mechanisms, as presented here, are evo-devo mechanisms inherent to the evolvability of Animalia, irrespective of an observable modification, but modification as a whole.…”

“…Brown [20] attributed the developmental program and associated genetic architecture as the governing body for phenotypic evolvability. In the absence of testable selection hypotheses [as is the case with Young et al [28], and likely many others], developmental constraints and release are the default governance of observable phenotypic differences. Providing both causal and mechanistic substance to evolvability, 'X' serves as the internal population features that relate to the probability of some change in an evolvability-based explanation.…”