Part 6: ‘Palaeo-pygmies’ and Dragon Man? (Part 5 Here)
The goals of this follow-on piece are to investigate ‘other’ commonly accepted Homo species and see if and how they may fit taxonomically and phylogenetically within a Homo evolutionary ‘tree’ or are just enmeshed within a reticulate ‘chain-link’ network.
By Cicero Moraes et alii (Luca Bezzi, Nicola Carrara, Telmo Pievani) | Wikimedia Commons
This pygmy-sized hominin is known only from a partial juvenile skeleton and a partial adult female skeleton. Since combined cranio-dental and post-cranial evidence suggests that sediba shares more evolutionarily derived features with Homo than any australopith, it was touted as the transitional form between Australopithecus and Homo, or even the sister species to the latter. However, despite derived, Homo-like aspects of the frontal lobe of its tiny brain, primarily because of sediba’s mosaic australopith/Homo morphology and small stature and cranium, it was initially placed within Australopithecus, closest to A. africanus.
This conclusion is ill-considered since, unlike africanus and like Homo, sediba has: a less pronounced brow ridge; its cheekbones are less flared; it has smaller teeth; and a less robust and prognathic (projecting-jaw) face with an “incipient” nose and a faint chin. Indeed, the combination of ancestral and derived traits in sediba well reflects a clear transition from a being adapted to partial arboreality in forest/woodland to one adapted for bipedal walking in savannas.
An alternative hypothesis vis-à-vis the evolution of its Homo-like face is that aspects of this resemblance can be attributed to the immaturity (neoteny) of the skull of the juvenile and that its ‘real’ derived craniofacial similarities are shared with A. africanus.
Post-cranially, sediba retained australopith-like, relatively long arms and elevated shoulder joints useful for climbing. The mosaic morphology of the hand also suggests the ability of strong flexion for climbing. However, other traits, especially the longer, more dexterous thumb and shorter fingers, indicate strong precision pinching associated with tool production. Its upper thorax is conical like in australopiths, but the relatively unflared lower thorax and vertebrae are more human-like. Its pelvic bones are australopith-like in dimensions involved with childbirth, but exhibit buttressing and orientation of the iliac blades that is more similar to the condition in Homo.
With regard to the leg, the proximal femur is australopith-like with a small head and long neck, but the bicondylar/carrying angle and insertion of the gluteus maximus are human-like. The knee exhibits both australopith- and human-like characteristics and, like modern humans, it was capable of fully extending its knees during the swing phase of walking.
The ankle bone is stout and more like those of non-human apes and features a medially twisted neck and a low-neck torsion angle. Indeed, the foot lacks the lateral plantar tubercle seen in humans and Australopithecus afarensis. The gracile heel bone and the robust malleolus (the bony prominence on each side of the ankle) are also quite apelike.
The resulting gait and related suite of adaptations exhibited by sediba are not found either in earlier hominins or any variant of Homo, and thus may have been ‘compromise’ between arboreality and habitual bipedalism. This suggests that there may have been more than one form of bipedalism during the Plio-Pleistocene. Therefore, considered holistically, sediba is either the most phylogenetically terminal taxon of the paraphyletic australopithecine ‘grade’ (see Part 3) or the basal lineage of Homo sensu lato. Furthermore, statistical tests based on quantitative evolutionary theory demonstrate that selection can be invoked to explain a rapid, adaptive, directional-selection-driven Australopithecus africanus–sediba–Homo transition co-occurring with substantial environmental change in Africa. Under this scenario, sediba could even have been ‘The Link’ – comfortable both in trees and open-country land – between australopithecines and early Homo.
Homo sapiens? naledi (Cradle of Humankind, South Africa, ca 335-236 Kya, ca 144 cm tall, brain volume 465–610 cc).
An exception to the norm that extinct Homo taxa are represented by only a small number of fossils from few individuals is the also pygmy-sized, moderately sexually dimorphic Homo naledi. More than 1,500 fossils representing at least 15 individuals of this species were deposited ca 236-335 Kya in the Rising Star cave system in South Africa.
Like sediba, naledi has a mosaic of ancestral australopith and derived Homo-like traits. For example, the shape and structure of its skull is similar to habilis and erectus and it is less prognathic than australopiths. Its teeth are small (especially canines, pre-molars and molars) and more similar in dimensions to modern sapiens and its molar crowns have five cusps – also like sapiens. However, like australopiths, it has: a small brain; a well-developed brow-ridge with a fissure stretching across just above the ridge and a pronounced occipital bun. Nevertheless, despite its small size, endocasts of naledi‘s brain share several aspects of structure in common with later forms of Homo, most notably in the organization of the inferior frontal and lateral orbital gyri.
Post-cranially, naledi’s shoulders, long arms, curved fingers/toes, flared hips and torso-configuration also present an australopith-like condition useful for climbing and hanging. But, its vertebrae, lower part of the pelvis (ischia), wrists, long thumb and foot are reminiscent of those in sapiens. Therefore, it probably was an obligate, but awkward, biped.
In summary, naledi probably represents a secondarily small, ‘retrograde’, independently derived pseudo-archaic species of Homo that co-existed with sapiens in Africa. However, given its relative evolutionary ‘youth’, it may also have retained its reproductive connectivity to sapiens sensu lato.
This third, pygmy-sized and chimp-brained Homo – nicknamed “Hobbit” – is based on fossils from 15 individuals. It inhabited the Liang Bua cave, going extinct soon after the arrival of modern sapiens. The Brodmann area 10 on the pre-frontal cortex (associated with cognition) of its brain is about the same size as that of modern humans.
On the one hand, the skull of floresiensis has an australopith-like low and anteriorly narrow vault shape. On the other, it exhibits laterally expanded parietals, a weak supra-meatal crest, a moderately flexed occipital, a marked facial reduction, and many other derived features that characterize post-habilis Homo.
Dental remains from multiple individuals indicate that floresiensis had australopith-like canine and premolars and more derived Homo-like molar morphologies.
The postcranial anatomy of floresiensis is that of a biped. However, although the bones and joints of the arm and shoulder are similar to early Homo, the lower limbs present a combination of ancestral and derived morphologies suggesting kinematic and biomechanical differences from the modern human bipedal gait, possibly primitive within the genus Homo. Its wrist bones are more similar to those of chimps and australopiths than to modern humans. Its foot is exceptionally long relative to the femur and tibia – proportions seen in some African apes. The hallux (big toe) is fully adducted, a condition that facilitates obligate bipedalism.
Overall, floresiensis is a palaeo-subspecies of Homo sapiens sensu lato probably derived from early Javanese erectus that, after ‘colonizing’ Flores, subsequently ‘de-volved’ – perhaps within 10 Ky – its secondarily small stature and brain size evolved via dramatic insular dwarfism.
As with the above-mentioned ‘Other’ hominins, luzonensis (nicknamed ‘Callao Man’) is pygmy-sized. It has been found only in Callao Cave and is based on 13 fossil bones (two fingers, three foot-bones, a thigh bone, and seven teeth) from two adults and a child.
Also like the preceding ‘other Homo’, luzonensis also shares the ‘normal potpourri’ of features with australopiths, erectus, sapiens and floresiensis, differing primarily in the morphology of its teeth and feet. Its premolars have characteristics similar to those seen in australopiths, habilis and erectus and have two or three roots, whereas those of sapiens have one or, in rare cases, two. The size and enamel/dentin of premolars are reminiscent of australopiths and early forms of Homo, but the molars are small like those of more recent variants of Homo.
The finger bones are long, narrow, and curved, reminiscent of australopiths and floresiensis. Like australopiths and habilis, they are dorso-palmarly (from the palm to the back of the hand) compressed, and have well developed flexor sheath attachment. The dorsal beak near the knuckle is strongly developed and angled towards the wrist rather than the finger.
The foot has both ancestral and advanced features, which points toward a distinctive way of walking that would seem counter-productive for modern humans. The base of each toe is substantially curved with signs of very developed muscle use to facilitate its bending.
Based on this extremely limited evidence and the fact that Luzon is well isolated from the Asian mainland and has a sizable endemic flora, like floresiensis, luzonensis at best qualifies as a palaeo-subspecies of H. sapiens sensu lato derived from an erectus ‘colonist’ that subsequently shrunk in size via insular dwarfism.
Homo longi (nicknamed Dragon Man) – represented by a single nearly complete skull with one molar – is from Late Middle Pleistocene deposits near Harbin City in north-eastern China. It was contemporary with several other tall human taxa that coexisted in Asia, Europe and Africa. It has been linked with a mandible and several other Middle-Late Pleistocene human fossils from elsewhere in eastern Asia that purportedly show mosaic/transitional combinations of features linking it to Chinese erectus and early sapiens. It is described and discussed in detail in a series of three research papers published in the open-access journal The Innovation.
Overall, Dragon Man exhibits yet another mosaic combination of ancestral and more recent features spanning the evolutionary history of Homo. The characters used to diagnose the skull are: a large (1,420 cc) cranium, flat face and small cheek bones – like sapiens, associated with more archaic erectus features – large square eye sockets, a massive brow ridge and a single over-large molar. Its low/long non-globular skull shape is similar to Middle Pleistocene Chinese erectus and neanderthalensis, but its low face with delicate cheekbones more closely resembles modern humans and its nose is rather large like Neanderthals.
Its name – Homo longi – is derived from “Long Jiang”, used commonly for the Heilongjiang Province. It literally means “dragon river”, hence the nickname “Dragon Man”.
Analyzed on its own, longi, is best considered a derived variant of East-Asian erectus, or even an exemplar of the morphologically enigmatic ‘denisova’.
Indeed, there are African erectus fossils that resemble longi. For example, an endocast of a ca 1 My, 995 cc human cranium from Buia, Danakil Eritrea is remarkably similar to longi. Although it is long and narrow when compared to it and erectus/ergaster sensu lato, its proportions are compatible with the morphology displayed by all archaic and medium-brained human-like hominins. The occipital areas display a pronounced ridge, the cerebellum is located in a posterior position, and the middle meningeal vessels are more developed in the posterior regions. These features are common among specimens attributed to Middle Pleistocene endocasts from eastern China, confirming a remarkable degree of morphological variability within erectus/ergaster.
Summarizing the taxonomy, phylogenetics and biogeography of the genus Homo and Homo sapiens sensu lato
The genus Homo is a 2.5-million-year-old offshoot of the African paraphyletic grade Austalopithecus spp. – ‘chimp-men’. Among them, it is closest to A. africanus. The pygmy-sized, small-brained Homo sediba is its most basal – and transitional – species, sharing some features with africanus. Homo habilis follows on cladistically. Thereafter, the cladistic structure of Homo becomes more reticulate (network-like) and its morphological and DNA evolution is best summarized by treating the remaining taxa as reproductively inter-connected chrono-spatial subspecies within a polytypic Homo sapiens sensu lato. H. s. erectus (‘ergaster’) evolved in – and diversified within – Africa, ultimately producing the secondarily pygmy-sized naledi and ‘giant’ sapiens sensu stricto. Soon after its emergence, erectus dispersed into Eurasia, producing the reproductively compatible large European neanderthalensis and Asian ‘denisova’/’longi’, with Asio-erectus independently colonizing Flores and Luzon and secondarily ‘de-volving’ into insular ‘dwarf-races’.
This narrative is depicted in the following chrono-cladogram.
Part 7 will attempt to navigate through an evidence-weak, mid-Pleistocene evolutionary ‘muddle in the middle’ that ultimately produced ‘modern’ – and invasive – Homo sapiens.