Human Origins
The earliest hominids arose in Africa approximately 6 million years ago?
The Human Family Tree
December 2000
Orriorin tugenensis
At the end of last year, there was a flurry of news stories about "Millennium Man," or the "Millennium Ancestor," a new hominid find from Kenya announced by Martin Pickford and and Brigitte Senut. They claimed that their new taxon was the oldest hominid yet described, from before 6 million years ago (Ma), and that it was fully bipedal.
Ardipithecus ramidus
This species was announced in September 1994. It is the oldest known hominid
species, dated at 4.4 million years. Most remains are skull fragments. Indirect
evidence suggests that it was possibly bipedal, and that some individuals
were about 122 cm (4'0") tall. The teeth are intermediate between those
of earlier apes and A. afarensis
Ardipithecus ramidus
White et al. have since discovered a skeleton which is 45%complete, on the basis of this evidence Ar. Ramidus is now considered a cousin group - also a possible ancestor for the genera Pan
Australopithecus anamensis
This species was only named in August 1995. The material consists of 9 fossils, mostly found in 1994, from Kanapoi in Kenya, and 12 fossils, mostly teeth found in 1988, from Allia Bay in Kenya (Leakey et al.1995). A. anamensis existed between 4.2 and 3.9 million years ago, and has a mixture of primitive features in the skull, and advanced features in the body. A partial tibia is strong evidence of bipedality, and a lower humerus is extremely humanlike.
Australopithecus afarensis
Used to be the oldest accepted hominid fossil (Lucy) from Hadar in Ethiopia is dated at 3.7 mya. Walked Upright, but had a small brain approximately the size of a chimpanzee
Australopithecus africanus
A larger animal than A. afarensis Both of these early species had pronounced sexual dimorphism with the males being larger than the females.
Paranthropus aethiopecus
In 1985, a cranium was found by Alan Walker at the west side of Lake Turkana in Northern Tanzania and was originally named Australopithecus aethiopicus. The cranium was as robust as any yet known, but was 2.5 million years old. This is thought to be the ancestor of the robust austropithecenes
Paranthropus boisei
Between 2 and 3 mya there was a diversification in the hominids. Two rather specialized larger animals Paranthropus. boisei and Paranthropus. robustus appear in the fossil record and slightly later fossils of a smaller but larger brained animal also appear - this is the first member of the genus Homo - Homo habilis.
P. boisei and P. robustus seem to have been rather specialized animals. They both have large heavy grinding jaws typical of herbivores
Homo habilis
Homo habilis seems to have been the first species to have made tools, all the Australopithecines used tools as do modern chimpanzees but Homo habilis is the first fossil hominid which is always associated with primitive stone tools.
Homo habilis is a collection of fossils that show a great deal of morphological variation there is a school of thought that thinks there were at least two Homo species at this time and we are lumping them together inappropriately.
In October 1993, an international team of paleontologists discovered a partial hominine mandible near Lake Malawi. The mandible was less robust than that in australopithecines and the cheek teeth smaller, indicating that it was closely associated with Homo. The authors named this specimen Homo rudolfensis, a contemporary of Homo habilis which was found at Lake Turkana. The Malawi hominid, together with other fauna that are characteristic of Eastern Africa, indicate significant fauna movement between the two regions.
Homo rudolfensis
Homo rudolfensis had a flatter, broader face and broader postcanine teeth with more complex crowns and roots, and thicker enamel. This species also had a larger cranium. All the non-australopithecine specimens found at Olduvai Gorge are known to be Homo habilis, whereas the ones found at Lake Turkana can be divided between Homo habilis and Homo rudolfensis.
Who is the Ancestor?
There is now a general agreement that two species of Homo coexisted 2 million years ago. Although the taxonomic distinction is based principally on cranial and dental characters, it is useful to think of Homo habilis as a smaller-brained creature with an archaic postcranium, and Homo rudolfensis as larger-brained with a more modern postcranium. Which of the two (if either) gave rise to later Homo is still being debated. Homo rudolfensis appears to have a good claim based on brain size and the more modern postcranium, but some insist that its facial and dental anatomy disqualify it from this role.
Homo ergaster
This hominid species was originally believed to be a different geographical population of Homo erectus, but it is now considered to be the ancestor of Homo erectus and Homo antecestor
Many aspects of Homo ergaster and Homo erectus anatomy are, of course, similar, with the principal differences being a higher cranial vault, thinner cranial bone, absence of sagittal keel, and certain cranial base characteristics in Homo ergaster. One distinguishing feature between early Homo and ergaster/erectus involves increased brain size (ranging between 850 and 1100 cc, with an increase over time), although the increase in body size actually means that the relative brain size has increased but little.
Other distinguishing features include a long, low cranium (particularly in Homo erectus), the presence of brow ridges, a shortened face, and a projecting nasal aperture, suggesting the first appearance of the typical human external nose with the nostrils facing downward. The structure of the nose would permit the condensation of moisture from exhaled air, which would have proved beneficial in a species that pursued an active subsistence strategy in warm, arid habitats, such as those occupied by early Homo ergaster.
Homo erectus
1.8-1.5 mya we find the first evidence of a larger brained group of fossils which are known as Homo erectus the stone tools associated with these fossils are more advanced than previously and the cranial capacity has increased. By 800,000 ya Homo erectus had migrated out of Africa and fossils are found all over the Old World.
Discovered by Kamoya Kimeu (one of the “Fossil Gang”) in 1984 at Nariokotome in Kenya. This is an almost complete skeleton of an 11 or 12 year old boy, the only major omissions being the hands and feet. It is the most complete known specimen of H. erectus, and also one of the oldest, at 1.6 million years. The brain size was 880 cc, and it is estimated that it would have been 910 cc at adulthood. The boy was 160cm (5'3") tall, and would have been about 185 cm (6'1") as an adult.
The recent redating "Java man" to 1.3 mya means that Homo erectus migrated out of Africa rather earlier than previously
Homo antecestor
Discovered in 1997 this fossil resembles Homo sapiens more closely than any fossil of similar age and is currently considered to be intermediate between the archaic Homo sapiens and Homo egaster
This species is often also referred to as "Archaic Homo Sapiens". Many examples of so-called Archaic Homo sapiens have been located, including some recent spectacular finds at Atapuerca, in North East Spain. These remains of many individuals include some that may be 780,000 years old. According to some proponents of the "Out of Africa" hypothesis, most of these specimens should be assigned to Homo heidelbergensis, which may have been ancestral to Neanderthals in Europe and to Homo sapiens in Africa. However, in May 1997, the discoverers of the fossils elected to name the fossils a new species, Homo antecessor. Multiregionalists view this group as evidence of a transition toward modern Homo sapiens.
Homo sapiens
The earliest fossils of Homo sapiens appear in North Africa 500,000 ya These fossils are not usually classed as Neanderthal but as an "archaic" form.
There is some dispute as to the position of the Neanderthals
Two main models
Out of Africa II - All modern humans are descended from a Homo sapiens that evolved in Africa and migrated out to displace Homo erectus In this scenario Neanderthals are advanced Homo erectus living in Europe - They seem to have been cold adapted and lived quite close to the edge of the ice sheet.
Multiregional - All modern Homo sapiens evolved from local groups of Homo erectus
This is highly contentious at present
The "Lucky Mother" - or "Eve' Hypothesis
Mitochondrial DNA analysis suggests all modern human mitochondria originated from one individual who lived in Africa 130,000 - 50,000 years ago.
Climate and human evolution
The opening of the Great Rift Valley over the last 4 million years and the climate changes 2-4 million years ago seem to be the driving forces in hominid evolution. First upright stance with its improved locomotory power and then intelligence were favored in this environment. P. boisei and P. robustus seem to have specialized on a declining resource as forest turned to savanna then to desert. Neanderthals also may have become too specialized and were out competed by the more flexible Cro-magnum.
The Development of Language
The final step in human evolution seems to have been the development of Language again a very contentious topic - however with this development humans left the realm of Darwinian Evolution for ever since through the power of language and culture we now pass information on without recourse to genes.
A New Fossil!!
In an article in March 22nd issue of Nature, Leakey and colleagues describe the new species Kenyanthropus platyops,which literally means "flat-faced man of Kenya," found in 3.5 million year old deposits in the Lomekwi drainage of Lake Turkana, some 350 miles north of the Tugen Hills. This taxon is contemporary with A. afarensis, and yet quite distinct from it, demonstrating that at least two different hominids coexisted in the Middle Pliocene.
