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Multilevel Human Evolution: Ecological Patterns in Hominin Phylogeny

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Multilevel Human Evolution: Ecological Patterns in Hominin Phylogeny doi 10.4436/jass.94026 JASs Proceeding Paper e-pub ahead of print Journal of Anthropological Sciences Vol. 94 (2016), pp. 167-182 Multi-level human evolution: ecological patterns in hominin phylogeny Andrea Parravicini & Telmo Pievani University of Padua, Department of Biology, Via U. Bassi n. 58/B, 35131 Padua, Italy e-mail: [email protected] Summary - Evolution is a process that occurs at many different levels, from genes to ecosystems. Genetic variations and ecological pressures are hence two sides of the same coin; but due both to fragmentary evidence and to the influence of a gene-centered and gradualistic approach to evolutionary phenomena, the field of paleoanthropology has been slow to take the role of macro-evolutionary patterns (i.e. ecological and biogeographical at large scale) seriously. However, several very recent findings in paleoanthropology stress both climate instability and ecological disturbance as key factors affecting the highly branching hominin phylogeny, from the earliest hominins to the appearance of cognitively modern humans. Allopatric speciation due to geographic displacement, turnover-pulses of species, adaptive radiation, mosaic evolution of traits in several coeval species, bursts of behavioral innovation, serial dispersals out of Africa, are just some of the macro-evolutionary patterns emerging from the field. The multilevel approach to evolution proposed by paleontologist Niles Eldredge is adopted here as interpretative tool, and has yielded a larger picture of human evolution that integrates different levels of evolutionary change, from local adaptations in limited ecological niches to dispersal phenotypes able to colonize an unprecedented range of ecosystems. Changes in global climate and Earth’s surface most greatly affected human evolution. Precisely because it is cognitively hard for us to appreciate the long-term common destiny we share with the whole biosphere, it is particularly valuable to highlight the accumulating evidence that human evolution has been deeply affected by global ecological changes that transformed our African continent of origin. Keywords - Multilevel theory of evolution, “Sahara pump” hypothesis, Mosaic evolution, Turnover pulses, Bursts of innovation, “Final wave” hypothesis. A multi-level approach to evolution and, according to Stephen J. Gould & Richard Lewontin (1979), this idea has been the domi- The evolutionary “Modern Synthesis” has nant “paradigm” in evolutionary biology for the provided a fundamental contribution in unifying greatest part of the twentieth century. population genetics with the ecological process of Modern Synthesis actually was not a mono- natural selection. It was based on the quite apri- lithic research programme. Some of the founding oristic idea that every macro-evolutionary phe- fathers – among others Theodosius Dobzhansky nomenon (i.e. any evolutionary change occur- (1937), Ernst Mayr (1963) and George G. ring at the species level and above) should have Simpson (1944) – recognized that some evidence been reducible to the micro-evolutionary level of (concerning paleontological, biogeographical, variations of gene frequencies in biological popu- and biological patterns) could be interpreted as lations, under selective pressures (Huxley, 1942). not strictly gradualistic as for the tempo and/ George Williams (1966) baptized “phyletic grad- or the mode of evolution. In order to account ualism” this kind of genetically based gradualism for the origin of species, Ernst Mayr proposed the JASs is published by the Istituto Italiano di Antropologia www.isita-org.com 168 Hominins in largest ecological framework the well-known pattern of allopatric specia- & Lieberman, 2014, p. 185). The micro- and tion, which matches genetic changes at popula- the macro- dimension of evolutionary processes tion level with environmental and geographical are two conceptually separated, but empirically macro-evolutionary triggers, factors of geograph- intertwined levels. The vertical dimension of ical isolation of populations and random genetic evolution (i.e. changes deriving from the trans- drift (Coyne & Orr, 2004). While from the gene- mission of selected genetic modifications) is centered perspective the evolutionary process strongly interrelated with the horizontal dimen- flows through “vertical” changes from micro- to sion of climatic and geographical factors, able macro-levels, in Mayr’s view speciation processes to produce episodic ecological changes which, are largely influenced also by “horizontal” dis- in turn, affect genetic and genealogical relations placements, i.e. geographical and climatic events. among organisms, populations and species. In other words, the genealogical continuity of According to the “hierarchy theory of evolu- change (i.e. variations of gene frequencies) inter- tion” proposed by Niles Eldredge, environmental acts with ecological instabilities and disturbances. events may have different magnitude: the greater In the 1970s the new discipline of paleobiol- is the magnitude, the greater ecosystems change; ogy (Sepkoski, 2012) contested through quanti- the greater the loss of higher taxa, the more dif- tative methods the ancillary role of paleontology ferent will be the newly evolved taxa. Eldredge in evolutionary studies, putting macroevolution compares his model to the “water sloshing in the at the center of the stage. The paleontological bucket - the size of the sloshes depending on how record was re-read in new ways, no longer as just hard the bucket is jolted” (Eldredge, 2008, p. the side-effect of the accumulation of micro-evo- 14). According to the “Sloshing Bucket model”, lutionary processes. Two Mayr’s pupils, Stephen the magnitude of the ecological perturbations J. Gould and Niles Eldredge (Eldredge & Gould, matches the extent of evolutionary change fol- 1972; Gould & Eldredge, 1993) developed lowing three main patterns: the “punctuated equilibria” theory, according a) When localized or sub-regional ecosystem to which a significant part of the evolutionary disturbances occur, most of the times con- change concentrates in rapid (geologically speak- specific populations which still live outside ing) events of speciation in peripheral isolates, the affected area replace the groups elimi- which undergo different processes (selection, nated by the physical disturbance, re-estab- drift, migration) that eventually produce the lishing similar local ecosystems. Little or clado-genetic pattern in allopatry that we see no perceptible evolutionary change occurs, in several phylogenies (Eldredge & Lieberman, and stasis prevails (Eldredge, 1999, 2003). 2014). In this view, speciation is the driver of b) When regional disturbances occur, they may major morphological changes. cause too rapid changes, ecosystems collapse Molecular evidence proved that a substan- and many species across different lineages tial percentage of evolutionary genetic change is become extinct. At this threshold level, most associated with bursts of punctuational branch- of the evolutionary change seems to occur, ing in phylogenies (Pagel et al., 2006; Pagel & in conjunction with the birth of new species, Venditti, 2014). Moreover, although recent stud- possibly triggering “turnover pulse” events. ies based on updated mathematical models estab- c) Global environmental disruptions give lished that sympatric and parapatric speciation place to mass extinctions, which affect the may frequently occur in nature too, allopatry still entire world’s biotas and result in the dis- may be considered the “null hypothesis” of spe- appearance of larger-scale taxonomic enti- ciation (Coyne & Orr, 2004). ties. At this level, modified species derive If so, macroevolution could be defined in from taxa that survived the mass extinction terms of “patterns and processes pertaining to the event, eventually giving rise to radiations of birth, death, and persistence of species” (Eldredge larger groups. A. Parravicini & T. Pievani 169 The two “walls” of the bucket represent two different hierarchies of levels. The genealogical hierarchy, concerning reproduction or replica- tion, involves genetically-based information systems: the micro-evolutionary level of genes is part of the upper level of organisms, which are nested into local breeding populations and species. The ecological hierarchy is about matter- energy transfer systems: organisms are parts of local conspecific populations seen in this case as “economic” entities acting for physical survival in their ecological niches. Local ecosystems are parts of regional ecosystems, up to encompass Fig. 1 - The “sloshing bucket” model (Eldredge, the whole biosphere. 2008, p.14). Organisms are simultaneously part of the two different interacting hierarchies: as repro- Following this trend of studies (Kingston, ducing “packages” of genetic information (rep- 2007; Pievani & Serrelli, 2013), the present the- licators), they are part of the genealogical hier- oretical review aims to apply a multi-level eco- archy; as matter-energy transfer systems (inter- logical approach to some major issues emerging actors), they are part of the ecological hierarchy in recent paleoanthropological researches. and their business is to survive. At the scale of local populations, reproducing groups and eco- logical groups often do not coincide. The pro- Climate instability and the first cess of natural selection at the level of organisms phases of hominin evolution incorporates the two dimensions as well (ecologi- cal pressures and genetic variations),
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