Part 4: ‘Hominin–ophilia’: discovering vs ‘inventing’ species of Homo OR just lots interbreeding with the one you’re with?
Genera, species or ‘kissing cousins’?
Part 3 in this series covered the evolutionary ‘smorgasbord’ and ‘tangled’ taxonomy that was Australopithecus: the African ‘chimp-men’ – not ‘Southern Apes’ as the name implies. Australopithecines were widespread throughout sub-Saharan Africa as far back as 3.5 million years ago (Mya). Homo emerged evolutionarily ‘instantly’ in Africa 2.5-2 million years ago (Mya), anagenetically from one (A. africanus/garhi/sediba?) or a ‘genetically reticulate amalgamation’ of multiple ‘gracile’ australopithecines. As outlined in Part 3 , during that interval, there could have been as many as nine species and/or subspecies (races) of australopithecines and proto-homo evolving (and interbreeding?) into a radically different lineage. Thereafter, depending on whether you’re an anatomist, palaeontologist, anthropologist or palaeo-geneticist and your species concept, there were/are as many as nine genera of archaic humans to as few as one and 19 to as few as one species of Homo.
However, before I try to deal with this disparity, I need to explain what constitute biological genera, species and subspecies AND humans.
In bird and mammal taxonomy, in practice, a genus is a species/group of species separated from its closest relatives by a “decided gap”. A “decided” gap is generated by possession of a combination of morphological, genetic, physiological, behavioural and/or ecological characters – often derived as a consequence of evolutionary radiation into a novel adaptive zone. In a cladogram (evolutionary genealogy) aimed at summarizing evolutionary divergence, the gap is depicted by a longer branch relative to those between component species within the genus. Within extant Great Apes (Family Hominidae), there are four genera: Pongo (orangutans), Gorilla (gorillas), Pan (chimps + bonobos) and Homo (humans). If one includes fossil chimp-men, straddling the gap between Pan and Homo, there are four more ‘australopithecine’ genera: Sahelanthropus, Ardipithecus, ‘gracile’ Australopithecus and ‘robust’ Paranthropus – covered in Part 3.
Elsewhere, I have outlined a range of species concepts in detail. Ideally, a species is a reproductively cohesive and genetically isolated lineage of ancestral-descendant populations that has identity. For sexual organisms, “reproductively cohesive” means possession of attributes that promote specific-mate recognition and assortative mating. “Isolated” means post-mating incompatibility that leads to death of embryos and/or offspring before reaching sexual maturity, or the inability of adults to produce viable offspring. “Identity” means possessing a unique or consilient combination of evolutionarily ancestral and novel diagnostic characters (qualitative, invariant features – morphological, genetic, behavioural and ecological).
Subspecies (‘races’) are reproductively compatible, normally geographically contiguous, evolutionarily significant groups of genetically open populations that are diagnosable by step or steep, congruent clinal (along a gradient) variation in several characters. Where subspecies distributional ranges meet, relatively narrow ‘suture’ zones attributable to interbreeding are characterized by morphologically intermediate and/or genetically heterogeneous individuals with ‘shuffled’, non-diagnosing features. Well-known and well-marked polytypic (multi-subspecies) African species are the Helmeted Guineafowl (Pharaoh’s ‘Chicken’ – Numida meleagris) and the Papio Baboons illustrated in Part 1.
Sadly (tragically in the case of humans), especially during the 18th through 20th Centuries, the subspecies category was biologically implemented irresponsibly. Animal taxonomists used it to ‘discover’ evolutionarily trivial geographical variation in birds and mammals. Racists exploited it to ‘identify’ subsets of humanity who could be ‘justifiably’ oppressed and/or enslaved’ (see here and here).
Relative to their chimp-bonobo (Pan spp.) closest evolutionary relatives and australopithecine linking taxa, Humans sensu stricto (Homo sapiens sapiens) are: tall/massive/sexually-monomorphic; broad-hipped; slow maturing; monogamous; concealed-ovulating; altricial; long-lived; ‘naked’; sweaty; short-strong-armed-throwing; obligate bipedal; highly efficient dispersing; round-headed; big-brained; flat-faced; big-nosed; small-jawed; small-‘reduced-canined’ toothed in U-shaped configuration; diverse-‘dieted’ and ‘chinned’ AND employ symbolic language.
In sharp contrast, within Homo, the shorter more ‘robust’ Neanderthals – with whom humans share 99.7 percent of their DNA, interbred and share adaptive components of their genomes – are primarily quantitatively distinct. They had: a flatter-shaped, larger cranium; a much more prominent brow ridge; larger nose; broader rib cage; larger shoulder, elbow, hip and ankle joints; broader hips; shorter forearms; thicker metacarpals; larger/thicker knee-caps; and shorter and flatter shin bones.
NB extant human population groups worldwide are 99.8% genetically similar.
Making the Homo ‘grade’
Handy Man had a more human-like skull, a much larger (>30%) brain than australopithecines and made and used simple stone tools. However, ‘his’ post-cranial anatomy was more similar to that of australopithecines, and he was not expertly bipedal.
The first uncontested, obligate bipedal species of still bigger-brained, pre-modern Homo is the highly polytypic (multi-subspecies) Afro-Asian erectus (‘split’ by some into “more primitive” African ergaster and derived Euro-Asian ‘true’ erectus). It emerged in Africa at ca 2.2 Mya and was anatomically decisively ‘closer’ to modern humans than to habilis.
Between 2 and < 1 Mya, Homo spread throughout – and evolved within – African open-country biotopes. By ca 1.5 Mya, anatomical H. habilis went extinct or was evo-genetically ‘assimilated’ within African erectus.
The marked temporal and geographical morphological variation within H. erectus sensu lato suggests that it possessed a high level of developmental plasticity – the ability to modify development in response to environmental conditions that is not genetically canalized – allowing it to occupy a variety of habitats.
While early Homo was spreading throughout open-country sub-Saharan Africa, there were several (many?) dispersal events involving erectus moving out of Africa into Eurasia. However, the key wave of immigration took root at ca 1.8 Mya. These small-brained (ca 550 cc) immigrants- -called H. georgicus by some ‘splitters – exhibited a mix of australopith/habilis-like and erectus-like characteristics. They had small, robust skulls and habiline-like, prognathic faces. Their upper limbs were australopith-like, whereas their spines and lower limbs were more homonine. This dispersal event seems to have been facilitated by the Saharan pump around 1.9 Mya, when the Sahara, Arabia and the Middle East were interconnected by savanna grassland. This allowed flora and fauna to move readily between Africa and western Asia.
By 1.2 Mya, erectus had settled across Europe through to China and subsequently sub-speciated into Neanderthals (extant ca 400-40 Kya) in Europe and west-Asia and Denisovans (300-50 Kya) in east-Asia. Neanderthals were particularly well-adapted to cold Pleistocene European climates. Their limb proportions appear to conform to Allen’s Rule (cold-adapted species tend to have short limbs), and their shape and relative robusticity conforms to Bergmann’s Rule (bodies tend to be rounder in colder climates). The anatomically still mysterious Denisovans appear to have straddled Central and East Asia and Southeast Asia and Oceania. These northern hemisphere hominins appear to have interbred freely when/where they came into contact. Indeed, there is genetic evidence of interbreeding between Homo sapiens and Neanderthals and Denisovans after humans spread from Africa into Eurasia ca 70 Kya. This has, led some palaeo-anthropologists to suggest – to my mind correctly – that forms of Homo, other than perhaps habilis, could be better described as subspecies, or palaeo-demes of a single, highly polytypic sapiens including the various forms of erectus.
‘Phony’ human genera and species?
Sadly, from the late-19th to mid-20th Centuries, there was a taxonomic ‘frenzy’ vis-à-vis early human fossils. At least eight ‘generic’ names (Pithecanthropus, Protanthropus, Sinanthropus, Cyphanthropus, Africanthropus, Telanthropus, Atlanthropus and Tchadanthropus) were assigned to bits and pieces of early human fossils. However, soon after my early mentor Ernst Mayr’s provocative 1950 paper, Taxonomic categories in fossil hominids, most were ‘submerged’ within ‘erectus’.
These are dealt with in Part 5.