Today 4 great apes still survive in Africa all of which share a more recent common ancestor to the exclusion of the Asiatic Pongo and appear to have diversified relatively recently from a common ancestor in Africa. The 4 African great apes display the following indisputable phylogenetic relationship:
(((Homo(Pan paniscus, P.troglodytes))Gorilla)Pongo)
Molecular studies strongly support a divergence of Homo from the Pan lineage at best around 4.5 Million years ago. In contrast the fossil hunters have been digging up an amazing array of fossil apes from Africa that have only resulted in more questions than answers with respect to affinities between various apes, their divergence times and their biogeography. Let us look at these apes and some issues:
-There are at least 3 representatives of genus Homo that nobody doubts are closer to extant Homo sapiens than any other great ape: 1) H.habilis, 2) H.erectus and archaic H.sapiens. There is the popular belief that H.habilis spawned the later H.erectus, which in turn spawned H.sapiens. Nevertheless, recent finds increasingly support the proposal where the two species overlap : H.habilis and H.erectus seem to have overlapped by around 500,000 years suggesting that they were sister species. Further, these finds like those based on the skull cap KNM-ER 42700 raise the possibility that H.erectus could have shown major sexual dimorphism, comparable to that seen in Gorilla. This is in contrast to H.sapiens where the dimorphism is low. Hence, it is possible that H.sapiens did not descend directly from H.erectus but archaic versions of H.sapiens were representatives of a sister lineage to H.erectus. Thus, we have the possibility that the divergence of these two Homo species was related to differential niche colonization and life-style divergence, rather than a linear model of one spawning the other. Finally, with respect to archaic H.sapiens there is the question of what is its relationship with modern early-branching African H.sapiens. None the less, in the absence of further fossils that situation is far from resolved.
-In addition to these problems, there is the issue of the relationship between African H.erectus and H.habilis with the early Homo from Dmanisi, Georgia. Some have taken these Eurasian specimens to imply a possible origin for early Homo in Eurasia followed by back migration to Africa.
–H.rudolfensis: Based on the absence of the characteristic supraorbital torus seen in H.habilis specimens like KNM ER 1813 or OH24 and a much large cranial capacity it was assumed that KNM ER 1470 is a different species of Homo from H.habilis. This species was termed H.rudolfensis and could potentially represent a precursor/sister group of H.sapiens, whereas H.habilis is exclusively allied with H.erectus (for example KNM ER 1813 might show an incipient transverse torus as seen in H.erectus). However, in the absence of further remains we cannot be sure of whether H.rudolfensis is indeed distinct or merely a part of the morphological variation spectrum shown by Homo of that period.
-The Australopithecines: The australopithecines clearly appear to be closer to Homo than to Pan. 3 major forms of australopithecines are recognized along with many other “controversial” forms. They are 1) A.afarensis 2) A.africanus and 3) the robust australopithecines, which are often given a separate genus Paranthropus and 3 “species” are recognized within them: P.aethiopicus, P.boisei, and P.robustus. Their robust flared skulls with enormous muscle attachment sites like the striking sagittal crest suggest a highly specialized ecology in terms of high fiber vegetable diet. More recently several other species have been named:
A.anamensis– this appears to be an form of the A.afarensis, currently going back to 4.1 MYA. It has been recognized a different species mainly due to its early age, but morphologically is within the observed variability of A.afarensis and probably represents the earliest members of that lineage. It is interesting to note that it is close to the molecular dates for the human-chimp split suggesting that chimp-human lineage inter-mating might have continued to about the time “A.anamensis” i.e. the earliest representatives of A.afarensis appeared.
A.bahrelghazali: this is the western-most specimen of an australopithecine ape, and all that is known of it is a single mandible that appears to be very close to A.afarensis. However, on the basis of a 3-rooted premolar it was made a separate species. Most likely it represents a poorly known western population of A.afarensis that might have at best undergone regional diversification.
A.garhi: this ape appears to be a larger version of A.afarensis. However, it was given a separate species status because of it is unusually large premolars and molars. It is also reported as having a crest, which is reminiscent of the robust australopithecines. Post-crania from the same site suggest a long femur, like Homo for this species. Its describers are fairly certain that it might represent a transitional form between A.afarensis and Homo. Given the scrappy nature of the finds it remains unclear if it might represent a distinct species of Australopithecus or a variant of A.afarensis.
The comparisons of jaw morphologies suggest that the well-established 3 australopithecine species share a derived jaw morphology, which appears to be convergently very close that of the Gorilla. The Homo lineage in contrast retains the primitive state. It has hence been proposed that the well-established australopithecine lineages may hence be precluded from human ancestry. In light of this, we need to see what the anatomy of A.garhi is, if it ever becomes better known, and whether it might be closer to Homo. But the take-home message is that convergence between related apes can mess up phylogeny, especially when we are dealing with such fragmentary remains.
–Kenyanthropus platyops: While described as a new ape contemporaneous with A.afarensis there has been much debate about its validity due to the poor preservation of the fossil. However, one should note that it has a small external acoustic porus compared to other australopithecines suggesting that it might indeed be a distinct lineage. Some have noted similarities with H.rudolfensis, but these may indeed be superficial and convergent. Yet again this is another case of a tantalizing fossil without a proper phylogenetic home.
–Ardipithecus, Orrorin and Sahelanthropus: These 3 apes appear to be the oldest of the apes belonging to the African great ape clade. All the 3 apes have been claimed to be in the human lineage to the exclusion of Pan. All three have been claimed to be erect bipedal walkers. One group has even gone as far as a to claim that all 3 are even the same genus of ape. However, they are all quite enigmatic as yet, and all of the above claims are definitely debatable and probably even incorrect. Ardipithecus ramidus, which was identified first was claimed to be on the human lineage, after its divergence from the chimps and the common ancestor of the Australopithecines from which Homo is eventually believed to have been derived. Subsequently a second species, Ardipithecus kadabba was described. Ar.ramidus was dated to about 4.5 MYA, whereas Ar.kadabba was dated as being 5.4-5.8 MYA. By the molecular criteria Ar.kadabba squarely falls in the period when the chimp and human lineages were still mating and probably only barely separated. Consistent with this Ar.kadabba is very chimp-like especially in its dental features, like the upper-canine lower premolar occlusion. This raises questions regarding whether the available features truly distinguish whether Ardipithecus belongs to the chimp to human lineage. Between Ar.kadabba and Ar.ramidus are we seeing gradual divergence or is it merely an issue of sample size?
Orrorin is dated to around 6 MYA is again fragmentary and poorly described. Sahelanthropus has recently been re-interpreted and definitely seems to belong outside of the Homo lineage. Based on molecular criteria both these apes seem to be members of common ancestral lineage of Pan and Homo. They either represent the common ancestor itself or radiations of that ancestral lineage. Most striking is the biogeography of Sahelanthropus — like “A.bahrelghazali” it is from central Africa suggesting that around 6 MYA the great apes of Africa were already well distributed over the continent. It also shows how little we know of the real spread of the African great ape clade. This has considerable consequences for inferences regarding the ancestral habitat of this clade, as well as the role of habitat, if any, in the emergence of the human lineage.
–Chororapithecus abyssinicus: The latest of the African great apes to be reported. It is a pathetic assemblage of a few gorilla-like teeth from a 10 MY old Chorora formation near Afar. The teeth surely look like a gorilla in gross as well as a certain subtle features of enamel-dentine ratios, but they are too little to say much, given the rampant convergence of various dental and mandibular features amongst great apes. All we can say is that there was a large gorilla-sized great ape around 10-11 MYA in Africa. This definitely fills in the “ape gap” of Africa and raises questions as to whether the African great apes are a re-invasion from Eurasia (after the Sivapithecus-Pongo lineage migrated there). After all this means African great apes could have emerged in Africa itself from a Dryopithecus-like form and only the Pongo lineage left to Eurasia.