Adaptive Radiations, Bushy Evolutionary Trees, and Relict Hominoids
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The RELICT HOMINOID INQUIRY 1:51-56 (2012) From the Editor ADAPTIVE RADIATIONS, BUSHY EVOLUTIONARY TREES, AND RELICT HOMINOIDS Jeff Meldrum* Department of Biological Sciences, Idaho State University, Pocatello, ID 83209-8007 Recent developments in paleoanthropology Shortly thereafter, the hypothesis further have promoted a shift in attitude toward the retreated when it was recognized that African question of relict hominoids. Over a half Homo erectus (now H. ergaster), a large- century ago, interpretations of the hominin brained human ancestor, had coexisted with fossil record were markedly different. Australopithecus (Paranthropus) bosei, a Deriving from the influential evolutionary parallel lineage of small-brained facially concept of competitive exclusion (Gauss, robust hominins that presumably eventually 1934), as applied to human evolution (Mayr, went extinct (Leakey & Walker, 1976). These 1950), it was deemed that only one species species display the expected ecological could occupy the hominin niche at any given reaction to a sympatric competitor, i.e. niche point in time. From this emerged the Single partitioning, involving diet, micro-habitat Species Hypothesis (Wolpoff, 1971). This divergence, and possibly also temporal hominin niche was associated with adaptations differentiation of resource use (Winterhalder, for habitual bipedalism, reduced canines, tool 1981). Stephen J. Gould (1976) made a use, and culture. The latter was thought prediction in his popular column in Natural perhaps most significant, because with culture History, stating: “We know about three and the plasticity of learning, a species could coexisting branches of the human bush [Homo conceivably broaden its niche space, further habilis, Homo erectus, and Australopithecus reducing the potential for sharing the bosei]. I will be surprised if twice as many landscape with other hominins (but see more are not discovered before the end of the Winterhalder, 1981). century.” In 1976, Washburn and Ciochon challenged Indeed, mounting discoveries accumulating the reach of the hypothesis and opined that it at a steady pace, reveal dozens of hominin was not until the emergence of Homo erectus species spanning a seven million year period that one species became so successful that all (see Tattersall, 1996, but see White, 2009). others were eliminated. They allowed that the The hominin phylogentic tree becomes preceding more “ape-like hominins,” i.e. the increasingly bushy with each additional australopithecines, offered a radiation of species. This proliferation of species is not contemporary coexisting species (see Lewin & merely an artifact of taxonomic “splitters” vs. Foley, 2004). “lumpers.” Martin (1990) has estimated that a *Correspondence to: Jeff Meldrum, Dept. Biol. Sci., Idaho State University, 921 S 8th Ave., Stop 8007, Pocatello, ID 83209-8007. email: [email protected]. © RHI JEFF MELDRUM 52 mere 3% of past taxonomic diversity in reveal primitive anatomies reminiscent of H. primate paleocommunities has been habilis or Australopithecus, again leading recognized and documented in the fossil some to propose a pre-erectus African origin record. Assuming the same holds true for for the species (Tocheri et al., 2007; Jungers et hominins, and taking a conservative tally of a al., 2009; Morwood and Jungers, 2009). This dozen extinct hominin species, according to raises even more questions over hypotheses Martin’s estimate there could conceivably be about the origins of H. floresiensis and its 400 species of hominin as yet unknown. arrival on the Indonesian island of Flores. Given the particulars of the inferred natural Australopithecines are presently only known history of large-bodied primates, and from Africa. Did a late australopithecine/ early especially the tendencies for generalized Homo disperse across Asia without leaving behavioral ecology of hominins, such a high any record of its passage? This is certainly figure for hominin diversity seems rather under serious discussion. unlikely (Arcadi, 2006). It emphasizes Another hominin potentially exhibiting a however, that the currently known fossil more recent persistence then previously record likely underestimates past diversity. recognized is Asian Homo erectus. Dating of The perennial discovery of new hominin Homo erectus sites at the extreme of its range species attests to that expectation. in Southeast Asia has produced ages of 30-50 In addition to this growing appreciation of ka, suggesting possible contemporaneity with the bushiness of the hominin tree, there are modern H. sapiens arriving in the region revelations of the ever more recent persistence (Swisher et al., 1996). These younger dates of a number of the branches or lineages within were seemingly contradicted by a later study; the tree. One of the most surprising however the older dates could be attributed to discoveries was the enigmatic “Hobbit” or reworked sediments at the site (Indriati et al., Homo floresiensis. This diminutive hominin 2011). Should the younger dates be unearthed on the Indonesian island of Flores substantiated, this would prove a hominin has been dated to as recent as a mere 18 ka example of relative biogeographic isolation (Brown et al., 2004). The recognition of this and survival that parallels the persistence of startling species even prompted the editor of the last Neanderthals. Nature to point out that since Homo The discovery of the Denisova hominins floresiensis survived until so very recent, it added another branch to the bush (Krause et was now more likely that stories of other so- al., 2010). The fragmentary fossils date to called mythical, human-like creatures, such as only 30 ka and were recovered from a cave the yeti are founded on grains of truth (Gee, site in southern Siberia, in the Altai Mountains 2004). He went on to acknowledge the near the Mongolian border. The completed possibility that the taxonomic and adaptive sequence of the Denisova hominin genome diversity of hominins was always high, has established this species as distinct from remained high until very recently, and might modern humans and Neanderthals (Reich et not be entirely extinguished. This was a al., 2010). The remains include remarkably notable concession reflecting a changing robust teeth and toe bones (Mednikova, 2011; attitude, although one generally not so openly Reich et al., 2010). The tooth, if correctly displayed. identified as a third molar, is as large as that of The justification of the attribution of the an australopithecine. Green, one of the lead “Hobbit” to the genus Homo has been researchers contemplated, "…you have to questioned due to its small brain-size and wonder if there were other populations that primitive aspects of its skeleton (Meldrum, remain to be discovered.” 2004). Recent studies of wrist and foot bones That Neanderthals and modern Homo BUSHY TREES 53 sapiens coexisted on the European continent documentary “Searching for the Ultimate for tens of thousands of years has long been Survivor.” While middle Pleistocene hominins recognized. During this extensive period of were large, the Goliath moniker is an overlap they remained separate and distinct exaggeration (see Ruff et al, 1997). Whether populations. The sequencing of the Homo heidlebergensis’ range encompassed Neanderthal genome has revealed minimal eastern Asia is debated (Lu et al., 2011). introgression between the species (Green et However, a specimen of pre-modern hominin al., 2010; Currat and Excoffier, 2011). While recovered from the site of Lishu, on display at much press has been directed to this limited, Peking University, has a preliminary date of even trivial, gene flow, of even greater 12-20 ka (Lu, personal communication). implication is the flip-side to this observation Therefore, an observer of the Asian landscape – that such genetically similar species of only 20 ka could potentially encounter any remained almost entirely distinct in spite of of a half dozen hominin species coexisting millennia of contact. there. A new cave site in the Altai Mountains has The implication of the recognized bushy produced additional Neanderthal fossils, at the hominin tree was a major theme developed in most easterly known extent of their range, a Nova documentary series “Becoming with preliminary dates of only 10-20 ka Human.” The final episode, which introduced (Reich, personal communication). This is less modern humans, was titled “Last Human than half the previous latest occurrence for Standing: Many human species once shared Neanderthals previously documented – 28 ka, the globe. Why do we alone remain?” possibly as young as 24 ka (Delson and Introductory remarks addressed the singular Harvati, 2006). The geographic range for circumstance of Homo sapiens’ solitary Neanderthals may has increased in another inheritance of the world. The producers’ direction as well, with archeological evidence explanation for this situation echoed the suggesting they occupied the subarctic earlier pronouncement of Washburn and northern extent of the Ural Mountains in Ciochon (1976) on the supremacy of Homo Russia, some 33 ka (Slimak et al., 2011). Only erectus, by suggesting that in this case, Homo the recovery of skeletal remains will confirm sapiens were so successful that all other this site as Neanderthal. The possibility of hominins were eliminated from the scene. Neanderthal persistence into the present has This assertion may prove as unfounded for been examined in the scientific literature by Homo sapiens as it was for Homo erectus a Porshnev (1974),