Dmanisi reconsidered

Implications of LD 50-1 jawbone and Spoor H. habilis study for ‘variable single species’ theory

In October 2013, Lordkipanidze and his colleagues reported the discovery of an adult skull from Dmanisi, Georgia. The fifth skull to be discovered at the site, it was complete and undeformed; it is the only known fully-preserved adult hominin skull from the early Pleistocene. They also put forward the radical suggestion that the various species often proposed for early African Homo (Homo habilis, Homo rudolfensis, Homo ergaster and Homo erectus) were all actually variants of the same species, and that early Homo was a single lineage which evolved over time without differentiating into multiple species. This conclusion is based on a claim that shape variation between the five Dmanisi skulls is roughly the same as that seen among the various early Homo skulls from East Africa, even though the former represents a single species and the latter are generally thought to represent several (Lordkipanidze, et al., 2013).

This suggestion must now be reconsidered in the light of last week’s announcement that the LD 50-1 partial lower jawbone places the origins of Homo at least 2.8 million years ago (Villmoare, et al., 2015); and Spoor and colleagues’ (2015) reappraisal of the OH 7 Homo habilis type specimen. The latter report that:

1.       The cranial capacity of OH 7 is estimated at between 729 and 824 cc, which is substantially larger than 500 to 700 cc typically cited and within the range of early Homo erectus;

2.       OH 7 is more primitive than the 2.33-million-year-old AL 666-1 upper jawbone provisionally assigned to Homo habilis, despite being 500,000 years younger.

Spoor and colleagues suggest that AL 666-1 cannot be placed within either Homo habilis or Homo rudolfensis. They do not investigate its true affinities further but suggest that their data is consistent with it belonging to Homo erectus. That it is more derived than Homo habilis implies that the origins of the latter must be sought even further back in time than 2.33 million years ago. The reporting that LD 50-1 is transitional between Australopithecus and Homo places the origin of the latter at around 2.8 million years ago. Putting the two reports together, the implication is that Homo was already diverse lineage by 2.33 million years ago, and that early human types were distinguished by facial morphology rather than by brain size.

Lordkipanidze and his colleagues noted that the morphological diversity within the five skulls recovered at Dmanisi is greater than that recorded for specimens recovered in Africa and assigned to different species. On the basis that the Dmanisi hominins all belong to the same species, they suggest that the morphological diversity in African fossil record of Homo 1.8 million years ago is better interpreted as demes of a single evolving lineage of Homo erectus rather than multiple species.

Given that Homo habilis, Homo rudolfensis and Homo erectus might all have emerged by 2.33 million years ago, an ancestor/descendant relationship between these species seems unlikely. It could be argued that the deme interpretation is more parsimonious than the traditional multiple species view.

Against this view, it has been suggested that the limb proportions of Homo habilis are more apelike (longer arms, shorter legs) than those of Homo erectus (Richmond, et al., 2002). The main problem is that in comparison to classic fossils such as ‘Lucy’ (Australopithecus afarensis) and the Turkana Boy (Homo erectus), we do not have a well-preserved example of Homo habilis. The evidence for the apelike limb proportions of Homo habilis largely rests on the two fragmentary skeletons KNM-ER 3735 and OH 62.

In the case of OH 62, the leg is represented by an upper section of the femur, the true length of which is uncertain. KNM-ER 3735 preserves even less limb detail. A re-evaluation of these specimens in comparison to fossil limb parts OH 34 and OH 35 suggests that the upper-to-lower limb ratio of OH 62 lies within the upper range of modern humans and lower range of chimpanzees; and that KNM-ER 3735 lies in the middle of the modern range, entirely outside the chimpanzee range. Based on these results, the limb proportions of Homo habilis were modern rather than apelike (Haeusler & McHenry, 2004).

Overall, these results appear to refute suggestions that Homo habilis should be reclassified as an australopithecine (Wood & Collard, 1999) and are consistent with the conclusions of Lordkipanidze and his colleagues.

References:

1.      Haeusler, M. & McHenry, H., 2004. Body proportions of Homo habilis reviewed. Journal of Human Evolution, Volume 46, pp. 433-465.

2.       Lordkipanidze, D. et al., 2013. A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo. Science, 18 October, Volume 342, pp. 326-331.

3.       Richmond, B., Aiello, L. & Wood, B., 2002. Early hominin limb proportions. Journal of Human Evolution, Volume 43, pp. 529-548.

4.       Spoor, F. et al., 2015. Reconstructed Homo habilis type OH 7 suggests deep-rooted species diversity in early Homo. Nature, 5 March, 7541(519), pp. 83-86.

5.       Villmoare, B. et al., 2015. Early Homo at 2.8 Ma from Ledi-Geraru, Afar, Ethiopia. Science, 5 March.

6.       Wood, B. & Collard, M., 1999. The Human Genus. Science, 2 April, Volume 284, pp. 65-71.

The LD 350-1 jawbone

Introduction:

A partial lower jawbone and a number of teeth have been recovered from a surface outcrop of fossil-bearing sedimentary rock in the Ledi-Geraru research area, in the Afar region of Ethiopia. This region has long been associated with the fossils of early hominins. The jawbone has been assigned to Homo (species indeterminate) (Villmoare, et al., 2015). The age of the jawbone is constrained by stratigraphic and palaeomagnetic considerations to between 2.80 and 2.75 million years old (DiMaggio, et al., 2015). This means that LD 350-1 is at least 400,000 years older than the earliest previously-known fossil assigned to Homo. The findings are published as two articles in the journal Science.

What was our previous understanding of human origins?

The conventional view is that the first human species was Homo habilis (‘Handy man’). Discovered in 1960 and announced as a new species four years later, it is believed to have evolved from an australopithecine ancestor though which is disputed. Possibilities include the South African Australopithecus africanus and Australopithecus afarensis from East Africa (or its probable descendant species Australopithecus garhi). The famous ‘Lucy’ belongs to Australopithecus afarensis. In comparison to a modern human, Homo habilis was small brained and its limb proportions (short legs, long arms) were still very apelike. However, the skull was less massively-built than an australopithecine; the upper and lower jawbones were within the human size range; and the feet and thumb joints were humanlike (Conroy, 1997). Homo habilis is known in the fossil record from 2.33 to 1.44 million years ago (Kimbel, et al., 1997; Spoor, et al., 2007). It is then presumed to have given rise to Homo erectus. Although still small-brained in comparison to a modern human, the limb proportions of Homo erectus are similar to those of later humans. Homo erectus first appears in the fossil record 1.8 to 1.9 million years ago (Wood, 2011).

This conventional view has a number of problems. Firstly, a second species, Homo rudolfensis is now known to have been contemporary with Homo habilis. First proposed in 1972, it was confirmed as a separate species in 2012 (Leakey, et al., 2012). How Homo rudolfensis fits into the bigger picture is far from clear: some have noted similarities to an earlier hominin, Kenyanthropus platyops (‘Flat-faced man of Kenya’) and it is possible that Homo rudolfensis belongs in Kenyanthropus rather than Homo.

Another problem is that the earliest example of Homo habilis, a 2.33 million year old upper jawbone known as AL 666-1 from Hadar, Ethiopia might in fact be something other than Homo habilis. The oldest uncontested example of Homo habilis is only 1.9 million years old (Lieberman, 2007) and given that the species also persisted well after the appearance of Homo erectus an ancestor/descendant relationship seems unlikely. Instead, it has been suggested that the two species shared a common ancestor (Spoor, et al., 2007).

Finally, it has been suggested that the late australopithecine species Australopithecus sediba from South Africa, which dates to around 2 million years old (Pickering, et al., 2011) is a more plausible ancestor for Homo erectus than is Homo habilis (Berger, et al., 2010).

Could LD 530-1 be an australopithecine?

The date of 2.8 million years ago puts it just after the time of Australopithecus afarensis (3.9 to 3.0 million years ago) and before the late australopithecine species Australopithecus garhi (2.5 million years ago).Given that the Australopithecus garhi is thought to descendant of Australopithecus afarensis, LD 350-1 is in the right place at the right time to be a part of that lineage. In terms of size, both the jawbone and the teeth are within the Australopithecus afarensis range, albeit towards the lower end. However, most other respects, the mandibular and dental characteristics of LD 350-1 fall outside the range for Australopithecus afarensis. The dentition is also reduced (and therefore more humanlike) in comparison to Australopithecus garhi, which would appear to bump the latter from the lineage leading to Homo. Overall, LD 350-1 appears to be transitional between Australopithecus and Homo and is likely to represent the earliest-known example of the latter.

What are the implications if LD 350-1 is indeed Homo?

Models that posit an australopithecine ancestor for Homo from the period 2.5 to 2.0 million years ago (e.g. Australopithecus garhi or Australopithecus sediba) would be ruled out. Instead, Homo diverged from Australopithecus much earlier than hitherto believed. Notably, 2.8 million years ago coincides with a shift to a more arid climate in Africa, suggesting a link between climate change and the emergence of Homo. The fossil record of Ledi-Geraru records a shift to a more open habitat of grasses or low shrubs at around this time.

Reboot for Homo habilis

In a separate study, published in the journal Nature, Fred Spoor and colleagues (Spoor, et al., 2015) carried out a reconstruction of the 1.8 million year old Homo habilis holotype specimen OH 7. The results suggest that the species was larger-brained than previously believed, within the range of Homo erectus. It was also found that the dentition of OH 7 is more primitive than the 2.33 million year old AL 666-1, suggesting that the latter cannot be Homo habilis – but implying that the origins of Homo habilis go back even further. The study did not consider the affinities of AL 666-1 any further but speculated that it could be early Homo erectus. By 2.33 million years ago, the Homo lineage was already apparently diverse, with early human species distinguished from one another more by gnathic morphology than by brain size. The reporting of the early Homo jawbone LD 350-1 dovetails neatly with this study.

What species is LD 350-1?

It is more primitive than Homo habilis but nevertheless lies within Homo. Its describers did not assign a species to it, but it is likely that it will be eventually recognised as a new species within Homo; the earliest human species yet. 

References:

1.      Berger, L. et al., 2010. Australopithecus sediba: A New Species of Homo-Like Australopith from South Africa. Science, 9 April, Volume 328, pp. 195-204.

2.       Conroy, G., 1997. Reconstructing Human Origins: A Modern Synthesis. New York, NY: W. W. Norton & Company, Inc..

3.       DiMaggio, E. et al., 2015. Late Pliocene fossiliferous sedimentary record and the environmental context of early Homo from Afar, Ethiopia. Science, 5 March.

4.       Kimbel, W., Johanson, D. & Rak, Y., 1997. Systematic Assessment of a Maxilla of Homo From Hadar, Ethiopia. American Journal of Physical Anthropology, Volume 103, pp. 235-262.

5.       Leakey, M. et al., 2012. New fossils from Koobi Fora in northern Kenya confirm taxonomic diversity in early Homo. Nature, 9 August, Volume 488, pp. 201-204.

6.       Lieberman, D., 2007. Homing in on early Homo. Nature, 20 September, Volume 449, pp. 291-292.

7.       Pickering, R. et al., 2011. Australopithecus sediba at 1.977 Ma and Implications for the Origins of the Genus Homo. Science, 9 September, Volume 333, pp. 1421-1423.

8.       Spoor, F. et al., 2015. Reconstructed Homo habilis type OH 7 suggests deep-rooted species diversity in early Homo. Nature, 5 March, 7541(519), pp. 83-86.

9.       Spoor, F. et al., 2007. Implications of new early Homo fossils from Ileret, east of Lake Turkana, Kenya. Nature, 9 August, Volume 448, pp. 688-691.

10.    Villmoare, B. et al., 2015. Early Homo at 2.8 Ma from Ledi-Geraru, Afar, Ethiopia. Science, 5 March.

11.    Wood, B., 2011. Did early Homo migrate “out of ” or “in to” Africa?. PNAS, 28 June, 108(26), p. 10375–10376.

Fifty years of Homo habilis

 The first human species - or was it?

Fifty years ago, the British anthropologist Louis Leakey and two colleagues reported the discovery of a new human species, Homo habilis (‘handy man’), in the journal Nature. Homo habilis lived at least 1.9 million years ago and remains the earliest-known widely-recognised human species to this day. The new species was described from fossils recovered at Olduvai Gorge, Tanzania between 1960 and 1964, but the story of its discovery began more than three decades earlier in 1931, when Leakey first investigated this now world-famous site.

Leakey believed that humans had evolved from African apes, as Darwin had originally suggested. By the early twentieth century however, this view had fallen out of favour and an Asian origin was widely favoured. The earliest-known human species at that time was Homo erectus, which had been discovered in late nineteenth century and was then known only in Asia. Although Neanderthals had been discovered some decades before that, Homo erectus was the first human species to be discovered that lived significantly before Homo sapiens and its brain was only around two-thirds the size of a modern brain.

However, in 1924, Australian anthropologist Raymond Dart had studied an apelike fossil found at a lime quarry at Taung, near Johannesburg, South Africa. He noted that the spinal column entered the skull through the centre rather than the back, suggesting that it was a biped and therefore a very early human – although its brain was no larger than that of a chimpanzee. Dart named it Australopithecus africanus (southern ape from Africa). The discovery switched the focus back to Africa, and in the decades that followed, australopithecines were also found in East and Central Africa. What was missing was a human ancestor intermediate between the australopithecines and Homo erectus.

Leakey became interested in Olduvai Gorge when fossilised human remains were found there, though ironically these later turned out to be a comparatively recent burial. Olduvai Gorge is probably the best-known fossil site in the world, and is now a UNESCO World Heritage site. A steep-sided ravine in Eastern Serengeti, it was formed when a stream carved its way through sedimentary rock, revealing seven main archaeological layers going back two million years. It was originally known as Oldoway Gorge; but it and Olduvai are mispronunciations of the local name Oldupai Gorge, which in turn comes from the Maasai word for the wild sisal plant growing in the gorge.

The 1931 expedition failed to discover any fossils, but a number of stone tools were found. These included a rudimentary stone chopping tool that was made by chipping flakes off a stone cobble to produce a weighty, sharp-edged cutting tool capable of cutting into animal carcasses. The tool is now on display in the British Museum, which at 1.8 million years old is the oldest object in the museum’s collection. Leakey classed the find as Oldowan, for the then still-current name Oldoway Gorge. The tool was found in Bed I, the lowest, earliest archaeological level at the site; more sophisticated stone hand-axes were found in higher, later levels. Leakey believed that the site recorded a sequence leading from the simple chopping tools in the lowest levels to the far more sophisticated tools in the higher Bed IV. The search was now on for the maker of the Oldowan tools, but Leakey’s work was interrupted by the breakup of his marriage and the outbreak of the Second World War.  

In 1951, he returned to Olduvai Gorge with his second wife, Mary, and in 1959, after several fruitless seasons, the Leakeys were finally rewarded with the discovery of the fossil skull of a young adult in the same archaeological layer that had yielded the stone cobble tool. It was small-brained and large jawed, with massive chewing teeth. The new species was designated Zinjanthropus boisei; ‘Zinj’ is an ancient Arabic word for the coast of East Africa, and the name also honours expedition sponsor Charles Boise. The skull was given the affectionate nickname of ‘Dear Boy’ by Mary Leakey. Now known as Paranthropus boisei, ‘Dear Boy’ belonged to an offshoot of the australopithecine lineage that is thought to have been an evolutionary dead end. Could this have been the maker of the cobble tools? It seemed doubtful.

The Leakeys were then joined in the field by their son Jonathan, and in November 1960 Jonathan and Mary found a lower jawbone with 13 teeth still in place, together with finger, hand and wrist bones. Over the next three years further fossils were recovered and analysed with the help of primatologist John Napier and anthropologist Phillip Tobias. They came from a species with a larger brain and smaller teeth than ‘Dear Boy’. Louis Leakey believed that this was this was the real toolmaker. The new species was announced in the journal Nature in April 1964 and given a name proposed by Raymond Dart – Homo habilis.

Compared to the australopithecines, the skull of Homo habilis was less massively-built, and the upper and lower jaws were within the size range of both Homo erectus and modern humans. The feet were humanlike, as were the thumb joints – but it was shorter in stature and much smaller-brained than a modern human. Males averaged 5 ft. 1 in and females 4 ft. 1 in; the brain size of around 600 cc was far less than the 1350 cc average for a modern human, or even the 750 cc human minimum proposed by British anthropologist Sir Arthur Keith in the late 1940s. This figure lies midway between the largest gorilla brain and the smallest modern human brain. Homo habilis was nevertheless significantly taller and bigger-brained than the australopithecines. However, the limb proportions were still apelike, with proportionately long arms and short legs, suggesting that Homo habilis retained some apelike tree-climbing abilities.

The Olduvai fossils are 1.8 million years old. Most remains are from East Africa; but the skull STW 53 from Sterkfontein, South Africa, may also be Homo habilis. The oldest tentative fossil evidence for Homo habilis to date is AL 666-1, a 2.33-million-year-old upper jawbone recovered at Hadar, Ethiopia, but the oldest uncontested Homo habilis remains are only 1.9 million years old. The most recent Homo habilis fossil currently known is a 1.44-million-year-old partial upper jawbone from Koobi Fora, Kenya. These dates – if both correct – imply that the species survived for almost a million years. Homo habilis is not known to have left Africa, but it has been suggested that it might have been the ancestor of Homo floresiensis, the so-called ‘hobbit people’ from the Indonesian island of Flores.

The Oldowan stone tool tradition associated with Homo habilis was the most primitive of all stone tool traditions. We now know that such tools were also made by some of the later australopithecines and might have a response to deteriorating climate as the Earth entered the current series of ice ages 2.5 million years ago. It is possible that as the climate deteriorated, preferred food types became unavailable and australopithecines added more meat to their diet. The increased need to butcher carcasses led to the development of stone tools. Early Homo erectus also used Oldowan tools before switching to the more advanced hand-axes seen in the upper levels at Olduvai Gorge. However, many Homo erectus groups, particularly in the Far East, persisted with the Oldowan stone cobble tools.

Plaster casts of the inside of Homo habilis braincases have shown that the sulcal and gyral patterns (ridges and furrows that give the human brain its wrinkled look) were more humanlike than apelike. The frontal and parietal lobes are enlarged, and that the Broca’s Area was expanded in comparison to the same region in australopithecines and modern apes. The frontal lobes, which control higher brain functions including planning and reasoning, are located at the front of the brain. Behind them, on the top and on each side of the brain are the parietal lobes, which carry out a wide range of functions including spatial awareness and the processing of sensory information.

Broca’s Area is named for nineteenth century physician Paul Broca who demonstrated a connection with speech. Damage leads to Broca’s aphasia, where patients are unable to speak in a grammatically correct manner. This suggests some linguistic abilities, though recent research shows that the Broca’s Area is also associated with tool-making. It is possible that its expansion was linked to enhanced tool-making skills as well as or possibly instead of the use of language.

However, the late australopithecines that made stone tools had brains no larger than their forebears, so tool-making alone doesn’t explain why Homo habilis needed a bigger, better brain. Bigger brains might sound like a good idea, but the same could be said of owning a Rolls-Royce. The problem in both cases is that they are expensive to run, and there is a pretty good case for trying to get by without. Brain tissue requires over 22 times as much energy as an equivalent amount of muscle tissue. In a modern human, the brain uses around 16 percent of the body’s energy budget despite making up just 2 percent of the body’s overall mass. While the energy costs of the smaller Homo habilis brain were less than those of a modern human brain, they were still considerable.

A possible answer is the social brain hypothesis, a theory which links the brain size of primates to the size of their social group. The enhanced brainpower is needed to keep track of the complex social relationships that are normal in many primate societies – not just human ones. Larger, more co-operative social groupings in Homo habilis society might have been an evolutionary response to the deteriorating climate and reduced availability of food.

Just where Homo habilis belongs in the human family tree remains contentious, even half a century after its discovery was announced. Even its membership of the human league is now questioned, with some seeing it as the anthropological equivalent of the now ex-planet Pluto and arguing that it should be reclassified as an australopithecine.

Although most textbooks describe Homo habilis as the ancestor of Homo erectus, the view has been called into question. Recent fossil finds indicate that Homo habilis persisted alongside Homo erectus for hundreds of thousands of year at Koobi Fora, Kenya. This makes it unlikely that the latter evolved from the former, and instead it has been proposed that both shared a common ancestor about two million years ago. If it were to turn out that the disputed AL 666-1 upper jawbone was something other than Homo habilis, then this scenario would become likely as the oldest examples of the two species would then be practically the same age. Another possibility is that the true ancestor of Homo erectus is the recently-discovered australopithecine species Australopithecus sediba. Australopithecus sediba lived in South Africa two million years ago, with limb proportions said to be more humanlike than Homo habilis.

Do the fossils assigned to Homo habilis even represent a single species? The variation in Homo habilis fossils is considerable and it has been suggested that these actually represent two species. Some examples have a broader, flatter face and larger teeth than others, and it is has been proposed that these be assigned to a second species known as Homo rudolfensis. On the other hand, it has been claimed that the degree of variation between skulls assigned to Homo habilis, Homo rudolfensis and Homo erectus is actually no more than that between five early Homo erectus skulls found at Dmanisi, Georgia, and it has accordingly been suggested that all three are actually the same species. The problem with this view is that it does not explain the more apelike limb proportions of Homo habilis.

It is this detail together with the small brain size that has led some to believe that Homo habilis should be reclassified as an australopithecine. Quite simply, it is too unlike Homo sapiens to be regarded as a human species. However, there is no consensus on the issue. Studies have been conducted to determine whether it can be included in Genus Homo on the basis of anatomical characteristics shared with other members, but these have proved inconclusive.

If Homo habilis individuals could somehow have been aware that they would one day become the subject of such deliberations, it is unlikely that they would have been overly concerned. It should not be forgotten that they might have existed for as long as a million years, which is a testament to their success as a species.

The ancestry of Homo floresiensis

Did ‘hobbit people’ of Flores evolve from Homo erectus or a more primitive hominin?

Homo floresiensis is an extinct Late Pleistocene hominin species known only from the Indonesian island of Flores. The type specimen LB 1 is a diminutive 30-year-old female who stood just 1.06 m (3 ft. 6 in.) tall. Nicknamed ‘Flo’, she had a cranial capacity initially estimated to be just 380 cc, comparable to that of an australopithecine. Her weight was estimated to be somewhere between 16 and 36 kg (35 and 79 lb.). Yet she was apparently human: she lacked the large back teeth of an australopithecine, the proportions of her facial skeleton were those of a human, and she appeared to be a humanlike fully-committed biped (Brown, et al., 2004; Morwood, et al., 2004).

The extremely small cranial capacity has been hotly debated since the species was first described in 2004. Some have claimed that Flo was a modern human suffering from microcephaly, a developmental disorder leading to a smaller brain (Jacob, et al., 2006; Martin, et al., 2006), but the majority reject this view and recognise Homo floresiensis as a new human species with a long, low cranial vault and other features characteristic of archaic humans (Argue, et al., 2006; Falk, et al., 2005; Falk, et al., 2007; Tocheri, et al., 2007; Lyras, et al., 2008).

Two principle theories have emerged as to the origin of these hominins. The first is that Homo floresiensis was a dwarf form of Homo erectus (approximate cranial capacity 1,000 cc) which underwent a dramatic reduction in size as a result of a phenomenon known as insular dwarfism. Animals living on an island where food is relatively scarce and predators are few or absent will ‘downsize’ over many generations, in order to reduce calorific requirements. What is actually happening is that evolution is favouring the smaller offspring in each generation. If predators do not pose a threat, any advantages in being large will be outweighed by poorer fuel-economy. What has to be questioned is whether insular dwarfism could lead to brain size reduction of the extent seen in Homo floresiensis.

The second theory is that Homo floresiensis is derived from a hominin species more primitive and smaller-brained than Homo erectus such as Homo habilis (approximate cranial capacity 600 cc) or even an australopithecine (approximate cranial capacity 400 cc). This second model implies that Homo erectus was not the first hominin species to leave Africa, contrary to the widely-accepted Out of Africa 1 hypothesis.

Two studies, one published in 2010 and the other earlier this year, have focussed on decreases in brain size. The first study considered the decreases in body mass and brain size that have been documented for a number of other primate lineages. Researchers tested putative ancestors for Homo floresiensis against these, using the high, medium and low estimates of its body mass. Results suggested that Homo erectus is only feasible as an ancestor for Homo floresiensis if the low estimate of 16 kg (35 lb.) is accepted. For the medium estimate of 24 kg (53 lb.), Homo habilis or the Dmanisi hominins are more feasible as ancestors. The high estimate is not compatible with any proposed scenario (Montgomery, et al., 2010).

The second study re-evaluated the brain size of Homo floresiensis using micro-CT scanning, and obtained an upwardly-revised estimate of 425 cc. This revised figure means that the degree of brain size reduction in relation to body mass is less, and therefore easier to explain. Nevertheless, if Homo erectus was the ancestor, the decrease in brain size is still too great to be explicable solely as a scaling downwards as body mass is downsized, and some other factor must have come into play. In an environment where food is scarce, and given that brain tissue is ‘expensive’ in metabolic terms, further reduction in brain size might have been advantageous. However, this could not be achieved without some loss of cognitive ability. The authors of the report suggested that cognitive abilities comparable to Homo habilis might have sufficed in an island habitat lacking dangerous predators (Kubo, et al., 2013).

While this ‘dumbing down’ scenario cannot be dismissed, it seems implausible. According to the widely-accepted ‘social brain hypothesis’ (Byrne & Whiten, 1988), the large primates of primates evolved in response to a need to predict the likely future social behaviour of their fellows, and base relationships upon these predictions. While Homo floresiensis would not have had to face dangerous predators, individuals would still need to interact with other group members. It is difficult to see that being less smart than one’s fellows could be anything over than a severe disadvantage, regardless of other circumstances.

I would therefore be inclined to the view that Homo erectus was not the ancestor of Homo floresiensis, and this view is supported by a number of studies considering the skeletal evidence. These have noted that while the cranial metrics were consistent with Homo erectus, the limb proportions of Homo floresiensis had more in common with Australopithecus garhi (Argue, et al., 2006) and the feet were a mosaic of primitive apelike and derived humanlike features. The big toe was fully in-line, albeit short, and the metatarsals followed a humanlike sequence in which the 1st (innermost) was the most robust (sturdily-built), followed by the 5th (outermost), then 4th, 3rd, and finally 2nd. The foot, though, was disproportionately long in comparison to that of a modern human; the lesser metatarsals (2nd to 5th) were long; and the outer toes were long and curved, unlike the short, straight toes of a modern human (Jungers, et al., 2009). The fact that the feet and limb proportions of Homo erectus were modern suggests that Homo floresiensis evolved from a species that was more primitive, such as Homo habilis.

References:

1. Brown, P. et al., A new small-bodied hominin from the Late Pleistocene of Flores, Indonesia. Nature 431, 1055-1061 (2004).

2. Morwood, M. et al., Archaeology and age of a new hominin from Flores in eastern Indonesia. Nature 431, 1087-1091 (2004).

3. Jacob, T. et al., Pygmoid Australomelanesian Homo sapiens skeletal remains from Liang Bua, Flores: Population affinities and pathological abnormalities. PNAS 103 (36), 13421–13426 (2006).

4. Martin, R. et al., Comment on ‘‘The Brain of LB1, Homo floresiensis’’. Science 312, 999b (2006).

5. Argue, D., Donlon, D., Groves, C. & Wright, R., Homo floresiensis: Microcephalic, pygmoid, Australopithecus, or Homo? Journal of Human Evolution 51, 360-374 (2006).

6. Falk, D. et al., The Brain of LB1, Homo floresiensis. Science 308, 624-628 (2005).

7. Falk, D. et al., Brain shape in human microcephalics and Homo floresiensis. PNAS 104 (7), 2513–2518 (2007).

8. Tocheri, M. et al., The Primitive Wrist of Homo floresiensis and Its Implications for Hominin Evolution. Science 317, 1743-1745 (2007).

9. Lyras, G., Dermitzakis, M., Van der Geer, A., Van der Geer, S. & De Vos, J., The origin of Homo floresiensis and its relation to evolutionary processes under isolation. Anthropological Science (2008).

10. Montgomery, S., Capellini, I., Barton, R. & Mundy, N., Reconstructing the ups and downs of primate brain evolution: implications for adaptive hypotheses and Homo floresiensis. BMC Biology 8 (9), 1-19 (2010).

11. Kubo, D., Kono, R. & Kaifu, Y., Brain size of Homo floresiensis and its evolutionary implications. Proceedings of the Royal Society B 280 (1760) (2013).

12. Byrne, R. & Whiten, A., Machiavellian Intelligence (Oxford University Press, Oxford, 1988).

13. Jungers, W. et al., The foot of Homo floresiensis. Nature 459, 81-84 (2009).

Homo habilis

Introduction:
Homo habilis ("handy man") is an early human species that lived between 2.33 to 1.44 million years ago and is quite possibly the earliest member of genus Homo, though acceptance of it being an ancestor to modern humans or indeed of it being a human species at all is not universal. The species was first described by Louis Leakey in 1964.

Fossil Record:
Homo habilis is known from fossils recovered from Olduvai Gorge, Tanzania; East Turkana, Kenya; Sterkfontein, South Africa; and Hadar in the Awash Valley of Afar Depression, Ethiopia.

The first specimen to be discovered, now known as OH7, comprising a partial cranium and mandible, was discovered by Jonathon and Mary Leakey at Olduvai Gorge on 4 November 1960 and has been dated to 1.75 million years old. Other findings from Olduvai Gorge include OH 24, a cranium discovered by Peter Nzube in 1968 and believed to be 1.8 million years old; OH 13 (partial cranium, mandible and maxilla, discovered by N. Mbuika in 1963, 1.66 million years old); OH 8 (foot, probably from the same individual as OH 7, discovered 1960) and OH 62 (partial skeleton, including upper and lower limbs, believed to be female, discovered by Donald Johanson and Tim White in 1986, 2.0 million years old).

KNM-ER 1805 and KNM-ER 1813 were both discovered at Koobi Fora, East Turkana. KNM-ER 1805 is a partial cranium, maxilla, and mandible, dated to 1.74 million years old. The finder and the date of discovery are unknown. KNM-ER 1813 is a cranium dated to 1.8 million years old discovered by K. Kimeu in 1973.

KNM-ER 42703, also from Koobi Fora, is a right maxilla with an estimated geological age of 1.44 million years. It was discovered by John E. Kaatho in 2000. This is the youngest specimen assigned to Homo erectus, and is notable for being younger than KNM-ER 42700: a small, well preserved calvaria with an estimated geological age of 1.55 million years that has been assigned to (African) Homo erectus.

The Sterkfontein specimens include the partial cranium STW-53 discovered by A.R. Hughes in 1976 and dated 1.5 – 2.0 million years old.

From Hadar comes the maxilla AL-666-1, discovered by William Kimball and dated to 2.33 million years old, the oldest specimen with affinities to Homo habilis.

Description:
Homo habilis had a cranial capacity of between 509-675cc (Cameron & Groves, 2004), considerably larger than its putative ancestors, the australopithecines (375-500cc); and that of present-day chimpanzees (400cc), but much less than that of modern humans. It had a prognathic face (jutting jaw), moderate brow-ridges; no saggital keeling (a raised area along the centre of the skullcap); reduced dentition relative to australopithecines; and proportionately long arms and short legs relative to modern humans. It had an ape-like conical ribcage and possibly retained the ape-like ability of arboreal locomotion.

There was a degree of sexual dimorphism in the species. OH 62 which, as noted above, is believed to have been female, was about 1m tall and probably weighed 30kg. Males were probably around 1.3m tall and weighed around 36kg.

Technology:
Homo habilis is generally associated with the Oldowan industry (or Mode I), named for Olduvai Gorge by Louis and Mary Leakey because they were first recognised there. The Oldowan is the earliest stone tool industry and examples dating to around 2.5 million years old are known from Gona, Bouri and Hadar in Ethiopia; from Lokalelei, West Turkana, Kenya dating to 2.4 million years old; from Koobi Fora dating to 1.9 million years old and from Olduvai Gorge dating to 1.8 million years old.

The Oldowan is associated with a number of hominin species and is therefore a toll-making tradition rather than a particular culture. In addition to Homo habilis, the tradition may be associated with Parantropus (“robust australopithecines”) and the “late” gracile species Australopithecus garhi. The latter, discovered in 1997, was found in association with large animal bones showing cut-marks from stone tools. Although no actual tools were directly associated with the fossils, it seems likely that A. garhi made and used stone tools. The remains have been dated to 2.5 million years old. In addition, the Oldowan survived Homo habilis and is associated with a number of later human species.

The Oldowan is characterised by very simple stone tools. The main types are choppers made from cobbles or angular blocks of stone; hammer-stones, which are unmodified chunks that show signs of having been used as hammers in tool manufacture; scrapers made from both cores and retouched flakes detached from cores. Although there is a considerable variety and other types such as discoids and polyhedrons are recognised, much of the variation can be explained in terms of differences in the nature of the raw material available. The overall strategy was likely a least-effort strategy to produce either flakes with sharp cutting edges or cores for chopping. It seems unlikely that the Oldowan toolmakers mastered the sophisticated stone reduction strategies seen in later tool-making traditions.

Evolutionary history of Homo habilis:
The accepted view of human evolution has become considerably more complicated over the last twenty years and it is now recognised that the traditional view of progressive evolution from australopithecine to H. habilis to H. erectus and finally to H. sapiens is at best an oversimplification.

The picture for Homo habilis was complicated in the 1980s by the discovery that fossils previously assigned to Homo habilis actually belonged to two species – Homo habilis and Homo rudolfensis, although it does now seem likely that the latter is an australopithecine-type upright ape, off the line of human evolution.

Homo habilis is believed to have evolved from an australopithecine species, but there is no consensus as to which of those currently known, if any. The traditional assumption that Homo ergaster (i.e. African Homo erectus) evolved from Homo habilis has recently been challenged by the discovery that the two species were sympatric (co-existing) in the Lake Turkana basin in Kenya for almost half a million years, implying that they must have occupied different niches and leading some to believe that the two species diverged from a common ancestor 2.3 million years ago rather than one evolving from the other.

On the other hand Homo habilis appears in the fossil record some 300,000 years before Homo ergaster. Unless earlier H. ergaster remains come to light, it seems likely that a proto-ergaster population split away from an earlier population of H. habilis and only later came into contact with a population of the ancestral species.

For now, then, it is still widely accepted that Homo habilis is ancestral to all later human species, including Homo sapiens.

References:

Cameron D & Groves C (2004): Bones, Stones and Molecules: “Out of Africa” and Human Origins, Elsevier Academic Press.

Leakey LSB, Tobias PV & Napier JR (1964): A New Species of Genus Homo from Olduvai Gorge, Nature No. 4927, 4 April 1964.

Scarre C (2005) (Ed): “The human past”, Thames & Hudson.

F. Spoor, M. G. Leakey, P. N. Gathogo, F. H. Brown, S. C. Anton, I. McDougall, C. Kiarie, F. K. Manthi & L. N. Leakey (2007): Implications of new early Homo fossils from Ileret, east of Lake Turkana, Kenya, Nature Vol 448 9 August 2007.

© Christopher Seddon 2009

Homo rudolfensis

Homo rudolfensis is a controversial taxon proposed in 1986 by Soviet anthropologist Valery Alekseyev for the old fossil hominin skull KNM-ER 1470. The skull is believed to be 1.9 million years old and was recovered in 1972 at Koobi Fora on the eastern shore of Lake Turkana (then Lake Rudolf), Kenya. The skull had previously been assigned to Homo habilis, but Alekseyev proposed reassigning it to a new species on the basis of morphological differences, most significantly a cranial capacity which, at 750cc, was significantly larger than that of H. habilis (approx 600-680cc). It also had a flatter, broader face and broader post-canine teeth (molars and premolars), with more complex crowns and roots, and thicker enamel. Subsequently other fossils including mandibles and cranial fragments, all located at Lake Turkana, were assigned to the new species and there is a roughly 50/50 split between H. habilis and H. rudolfensis remains at this site.

Cameron & Groves (2004) reject Homo status altogether for KNM-ER 1470. By phylogenetic analyses of hominin craniofacial morphology, they demonstrated that the australopithecine-like species Kenyanthropus platyops (“flat-faced man of Kenya”), a contemporary of Australopithecus afarensis (“Lucy”), was probably ancestral to KNM-ER 1470 and suggested that Homo rudolfensis should be reclassified as Kenyanthropus rudolfensis.

Homo rudolfensis’ claims as a possible ancestor to modern humans received a further blow in 2007 when KNM-ER 1470 was reconstructed by anthropologist Timothy Bromage of the New York University. Bromage noted that the jaw had been positioned incorrectly and actually jutted out further than originally believed. He downsized the cranial capacity to 526cc, below that of Homo habilis. The reconstructed skull more closely resembles those of Australopithecus (gracile australopithecines) or Paranthropus (robust australopithecines) than those of Homo (humans).

Dr. Bromage’s conclusions are not universally accepted, but if correct they would reinforce the outcome of the Cameron & Groves study.

References:

Bromage T (2007): Craniofacial Architectural Constraints Resolve Major Quandry of Human Evolution, presentation to NYUCD, released 26 March 2007.

Cameron D & Groves C (2004): Bones, Stones and Molecules: “Out of Africa” and Human Origins, Elsevier Academic Press.

© Christopher Seddon 2009