Hybridization in human evolution: insights from other organisms

Abstract

During the late Pleistocene, isolated lineages of hominins exchanged genes thus influencing genomic variation in humans in both the past and present. However, the dynamics of this genetic exchange and associated phenotypic consequences through time remain poorly understood. Gene exchange across divergent lineages can result in myriad outcomes arising from these dynamics and the environmental conditions under which it occurs. Here we draw from our collective research across various organisms, illustrating some of the ways in which gene exchange can structure genomic/phenotypic diversity within/among species. We present a range of examples relevant to questions about the evolution of hominins. These examples are not meant to be exhaustive, but rather illustrative of the diverse evolutionary causes/consequences of hybridization, highlighting potential drivers of human evolution in the context of hybridization including: influences on adaptive evolution, climate change, developmental systems, sex-differences in behavior, Haldane’s rule and the large X-effect, and transgressive phenotypic variation.

Mixed-species associations and attempted mating suggest hybridization between purple-faced and tufted gray langurs of Sri Lanka

“…In nearly all cases, hybridization has been inferred based on putative hybrid offspring with intermediate coat color or observed interspecific sexual behavior. In several cases, genetic data have also confirmed the hybrid status of individuals (Zinner et al 2011 ; Ackermann et al 2019 ).…”

“…More recently, genetic analyses have confirmed greater levels of heterozygosity in purported hybrid zones relative to areas where species are reproductively isolated (Cortés-Ortiz et al 2007 ; Pastorini et al 2009 ; Charpentier et al 2012 ; Matsudaira et al 2013 ; Malukiewicz et al 2015 ). Molecular studies have also revealed ample evidence for ancient hybridization events across the primate lineage (Zinner et al 2009 ; Roberts et al 2010 ; Roos et al 2011 ), including in recent hominin evolution (Ackermann et al 2019 ), suggesting that many lineages have experienced gene flow throughout their evolutionary history. These studies underscore the emerging view that hybridization has played a key role in the evolution and extinction of lineages within the order Primates (Ackermann et al 2019 ; Cortés-Ortiz et al 2019 ).…”

“…Molecular studies have also revealed ample evidence for ancient hybridization events across the primate lineage (Zinner et al 2009 ; Roberts et al 2010 ; Roos et al 2011 ), including in recent hominin evolution (Ackermann et al 2019 ), suggesting that many lineages have experienced gene flow throughout their evolutionary history. These studies underscore the emerging view that hybridization has played a key role in the evolution and extinction of lineages within the order Primates (Ackermann et al 2019 ; Cortés-Ortiz et al 2019 ).…”

Revolutionary Fossils, Ancient Biomolecules, and Reflections in Ethics and Decolonization: Paleoanthropology in 2019

“…Combining evidence from multiple hybridizing organisms, Ackermann et al. (2019) present a case that emphasizes the significance of introgression for generating novel genetic and resultant phenotypic variability across lineages, with specific reference to the application of this comparative approach for understanding the possible evolutionary effects this could have had, both positive and negative, during hominin evolution. Evidence for genetic effects was provided by Hsieh et al.…”

“…The approach of this integrative analysis combined with recent studies evaluating the morphological effects of hybridization on phenotypic variability in extant organisms (e.g., Ackermann et al. 2019) lays the groundwork for further analyses on the effects of hybridization between hominin groups.…”