Paranthropus boisei or Australopithecus boisei was an early hominin, described as the largest of the genus Paranthropus. It lived in Eastern Africa during the Pleistocene epoch from about 2.4 until about 1.4 million years ago. First discovered by anthropologist Mary Leakey on July 17, 1959, at Olduvai Gorge, the well-preserved cranium was dated to 1.75 million years ago and had characteristics distinctive of the robust australopithecines. Mary and her husband Louis Leakey classified the find as Zinjanthropus boisei: "Zinj" for the medieval East African region of Zanj, "anthropus" for "human being", "boisei" for Charles Watson Boise, the anthropologist team’s benefactor. Paranthropus boisei proved to be a treasure when the anthropologists' son Richard Leakey suggested it was the first hominin species to use stone tools. Bernard Wood of the University of Liverpool, posits that tools discovered in Ethiopia and dated to 2.5 million years ago could have been made by Paranthropus boisei. A well-preserved jaw, known as the Peninj Mandible, was found by Richard's colleague Kamoya Kimeu in 1964 in Peninj, Tanzania.
Another skull, specimen KNM ER 406, was unearthed in 1969 by Richard at Koobi Fora near Lake Turkana, in Kenya. The brain volume is quite small, about 500 to 550 cm3, not much larger than that of Australopithecus afarensis and Australopithecus africanus or modern-day chimpanzees. In P. boisei the foramen magnum is much shorter than in P. robustus. In addition, the cranial variation of P. boisei is remarkably high. The adult males were larger on average than females, as was the case in all australopithecine species. Males weighed some 49 kg and stood about 1.37 m tall, while females weighed about 34 kg and were 1.24 m tall. It had a skull specialized for heavy chewing and several traits seen in modern-day gorillas; the molar teeth were large, with an area over twice that of modern humans. The species is sometimes referred to as "Nutcracker Man" because it had the biggest, flattest cheek teeth and the thickest enamel of any known hominin. P. boisei had large chewing muscles attached to a pronounced sagittal crest.
To accommodate their enormous cheek teeth, P. boisei had a wide face, although the front teeth were much smaller than in similar species. Some argue that the craniodental morphology of this taxon is indicative of a diet of hard or tough foods such as ground tubers and seeds. However, research on the molar microwear of P. boisei has found a pattern different from that observed in P. robustus in South Africa, thought to have fed on hard foods as a fallback resource. This work suggests. Carbon isotope ratios suggest P. boisei had a diet dominated by C4 vegetation, unlike P. robustus in South Africa. In 1993, A. Amzaye found fossils of P. boisei at Ethiopia. The partial skull is dated to 1.4 million years ago. It is the biggest skull specimen found of P. boisei. The oldest specimen of P. boisei was found in Omo and dates to 2.3 million years ago, classified as, while the youngest specimen from Olduvai Gorge dates to 1.2 million years ago and is classified as OH 3 and OH 38. OH 5 Zinjanthropus, "Zinj" or "Nutcracker Man", was the first P. boisei specimen found by Mary Leakey at Olduvai Gorge, Tanzania.
The name "Nutcracker Man" may be a misnomer as it may only apply to Paranthropus robustus." KNM ER 406 is a small partial cranium discovered by Richard Leakey and H. Mutua in 1969, found at Koobi Fora, which displays large zygomatic arches and a cranial capacity of 510 cm³. KNM WT 17400 is a partial cranium, with similar characteristics to KNM WT 17000, "Black skull", belonging to Paranthropus aethiopicus, was found at West Turkana, Kenya. Peninj Mandible is a well-preserved jaw, found by Kamoya Kimeu in the Lake Natron region, near the Peninj River in Tanzania. All primates and hominids are or were dietary generalists, meaning that they ate a wide variety of foods and did not depend on a single, specific food source; the most accepted theory about what foods P. boisei ate suggests that it fed on hard objects as its primary source of nutrition. Strong jaw muscles are believed to be an evolutionary adaptation of P. boisei for a diet of nuts and hard fruit. Analysis of the wear on the molars of P. boisei showed fine striations indicating constant chewing of small, hard food items, like nuts and seeds.
The foods that P. boisei would have ingested can be determined by means of carbon isotope measurements. Along with fruits and nuts, carbon isotope data on P. boisei fossils suggest that their diet was based on C4 resources grasses or sedges. One study has concluded that the diet of P. boisei consisted of grasses and sedges such as tigernut and that it ate fruits and nuts. According to another, the addition of tubers to its diet helped P. boisei to meet its daily caloric intake requirement. In any case, it would appear that P. boisei consumed larger quantities of plant matter than any other hominin studied to date. The diet of fruits and nuts that P. boisei would have eaten is reflected by its classification as a cosmopolitan species, meaning that its wide geographic range
The Denisovans or Denisova hominins are an extinct species or subspecies of archaic humans in the genus Homo. Pending its taxonomic status, it carries temporary species or subspecies names Homo denisova, Homo altaiensis, Homo sapiens denisova, or Homo sp. Altai. In 2010, scientists announced the discovery of an undated finger bone fragment of a juvenile female found in the Denisova Cave in the Altai Mountains in Siberia, a cave, inhabited by Neanderthals and modern humans; the mitochondrial DNA of the finger bone showed it to be genetically distinct from Neanderthals and modern humans. The nuclear genome from this specimen suggested that Denisovans shared a common origin with Neanderthals, that they ranged from Siberia to Southeast Asia, that they lived among and interbred with the ancestors of some modern humans, with about 3% to 5% of the DNA of Melanesians and Aboriginal Australians and around 6% in Papuans deriving from Denisovans. Several additional specimens were subsequently characterized.
A comparison with the genome of another Neanderthal from the Denisova cave revealed local interbreeding with local Neanderthal DNA representing 17% of the Denisovan genome, evidence of interbreeding with an as yet unidentified ancient human lineage, while an unexpected degree of mtDNA divergence among Denisovans was detected. The lineage that developed into Denisovans and Neanderthals is estimated to have separated from the lineage that developed into "anatomically modern" Homo sapiens 600,000 to 744,000 years ago. Denisovans and Neanderthals significantly diverged from each other genetically a mere 300 generations after that. Several types of humans, including Denisovans and related hybrids, may have each dwelt in the Denisova Cave in Siberia over thousands of years, but it is unclear whether they co-habitated in the cave; the Denisova Cave is in south-western Siberia, Russia in the Altai Mountains near the border with Kazakhstan and Mongolia. It is named after a Russian hermit who lived there in the 18th century.
The cave was explored in the 1970s by Russian paleontologist Nikolai Ovodov, looking for remains of canids. In 2008, Michael Shunkov from the Russian Academy of Sciences and other Russian archaeologists from the Institute of Archaeology and Ethnology of Novosibirsk investigated the cave, they found the finger bone of a juvenile hominin referred to as the "X woman", or the Denisova hominin. Artifacts excavated in the cave at the same level were dated using radiocarbon and oxygen isotopes to around 40,000 BP. Excavations have since revealed human artifacts showing an intermittent presence going back 125,000 years. A team of scientists led by Johannes Krause and Svante Pääbo from the Max Planck Institute for Evolutionary Anthropology in Leipzig, sequenced mtDNA extracted from the fragment; the cool climate of the Denisova Cave preserved the DNA. The average annual temperature of the cave is 0 °C, which has contributed to the preservation of archaic DNA among the remains discovered; the analysis indicated that the Denisova hominin "diverged from a common ancestor well before Neanderthals and modern humans did"—around 1 million years ago.
The mtDNA analysis further suggested that this new hominin species was the result of an earlier migration out of Africa, distinct from the out-of-Africa migrations associated with modern humans, but distinct from the earlier African exodus of Homo erectus. Pääbo noted that the existence of this distant branch creates a much more complex picture of humankind during the Late Pleistocene; this work shows that the Denisovans were a sister group to the Neanderthals, branching off from the human lineage 550,000 years ago, diverging from Neanderthals in the Middle East, 300,000 years ago. A second paper from the Svante Pääbo group reported the prior discovery of a third upper molar from a young adult, dating from about the same time; the tooth differed in several aspects from those of Neanderthals, while having archaic characteristics similar to the teeth of Homo erectus. They performed mtDNA analysis on the tooth and found it to have a sequence somewhat similar to that of the finger bone, indicating a divergence time about 7,500 years before, suggesting that it belonged to a different individual from the same population.
So far, the fossils of four distinct Denisovans from Denisova Cave have been identified through their DNA: Denisova 2, Denisova 3, Denisova 4, Denisova 8. Analysis of a fifth specimen, Denisova 11, proved it to have belonged to an F1 Denisovan-Neanderthal hybrid. Denisova 2 and Denisova 3 are prepubescent or adolescent females, while Denisova 4 and Denisova 8 are adult males. MtDNA analysis of the Denisovan individuals suggests the Denisova 2 fossil is the oldest, followed by Denisova 8, while Denisova 3 and Denisova 4 are contemporaneous. During DNA sequencing, a low proportion of the Denisova 2, Denisova 4 and Denisova 8 genomes were found to have survived, but a high proportion of the Denisova 3 genome had survived. Little is known of the precise anatomical features of the Denisovans, since the only physical remains discovered thus far are the finger bone, two teeth from which genetic material has been gathered, a toe bone; the single finger bone is unusually broad and robust, well outside the variation seen in modern people.
It belonged to a female, indicating that the Denisovans were robust similar in build to the Neanderthals. The tooth does not share the derived morphological features seen in Neanderthal or modern human teeth. An initial morphological
Neanderthals are an extinct species or subspecies of archaic humans in the genus Homo, who lived within Eurasia from circa 400,000 until 40,000 years ago. The earliest fossils of Neanderthals in Europe are dated between 450,000 and 430,000 years ago, thereafter Neanderthals expanded into Southwest and Central Asia, they are known from numerous fossils, as well as stone tool assemblages. All assemblages younger than 160,000 years are of the so-called Mousterian techno-complex, characterised by tools made out of stone flakes; the type specimen is Neanderthal 1, found in Neander Valley in the German Rhineland, in 1856. Compared to modern humans, Neanderthals were stockier, with bigger bodies. In conformance with Bergmann's rule, as well as Allen's rule, this was was an adaptation to preserve heat in cold climates. Male and female Neanderthals had cranial capacities averaging 1,600 cm3 and 1,300 cm3 within the range of the values for anatomically modern humans. Average males stood around females 152 to 156 cm tall.
There has been growing evidence for admixture between Neanderthals and anatomically modern humans, reflected in the genomes of all modern non-African populations but not in the genomes of most sub-Saharan Africans. This suggests that interbreeding between Neanderthals and anatomically modern humans took place after the recent "out of Africa" migration, around 70,000 years ago. Recent admixture analyses have added to the complexity, finding that Eastern Neanderthals derived up to 2% of their ancestry from anatomically modern humans who left Africa some 100,000 years ago. Neanderthals are named after one of the first sites where their fossils were discovered in the mid-19th century in the Neander Valley, just east of Düsseldorf, at the time in the Rhine Province of the Kingdom of Prussia; the valley itself was named for Joachim Neander, Neander being the graecicized form of the surname Neumann. The German spelling of Thal "Valley" was current in the 19th century. Neanderthal 1 was known as the "Neanderthal cranium" or "Neanderthal skull" in anthropological literature, the individual reconstructed on the basis of the skull was called "the Neanderthal man".
The binomial name Homo neanderthalensis—extending the name "Neanderthal man" from the individual type specimen to the entire group—was first proposed by the Anglo-Irish geologist William King in a paper read to the British Association in 1863, although in the following year he stated that the specimen was not human and rejected the name. King's name had priority over the proposal put forward in 1866 by Homo stupidus. Popular English usage of "Neanderthal" as shorthand for "Neanderthal man", as in "the Neanderthals" or "a Neanderthal", emerged in the popular literature of the 1920s. Since the historical spelling -th- in German represents the phoneme /t/ or /tʰ/, not the fricative /θ/, standard British pronunciation of "Neanderthal" is with /t/; because of the usual sound represented by digraph ⟨th⟩ in English, "Neanderthal" is pronounced with the voiceless fricative /θ/, at least in "layman's American English". The spelling Neandertal is seen in English in scientific publications. Since "Neanderthal", or "Neandertal", is a common name, there is no authoritative prescription on its spelling, unlike the spelling of the binominal name H. neanderthalensis, predicated by King 1864.
The common name in German is always invariably Neandertaler, not Neandertal, but the spelling of the name of the Neander Valley itself has been affected by the species name, the names of the Neanderthal Museum and of Neanderthal station persisting with pre-1900 orthography. Since the discovery of the Neanderthal fossils, expert opinion has been divided as to whether Neanderthals should be considered a separate species or a subspecies relative to modern humans. Pääbo described such "taxonomic wars" as unresolveable in principle, "since there is no definition of species describing the case." The question depends on the definition of Homo sapiens as a chronospecies, in flux throughout the 20th century. Authorities preferring classification of Neanderthals as subspecies have introduced the subspecies name Homo sapiens sapiens for the anatomically modern Cro-Magnon population which lived in Europe at the same time as Neanderthals, while authorities preferring classification as separate species use Homo sapiens as equivalent to "anatomically modern humans".
During the early 20th century, a prevailing view of Neanderthals as "simian", influenced by Arthur Keith and Marcellin Boule, tended to exaggerate the anatomical differences between Neanderthals and Cro Magnon. Beginning in the 1930s, revised reconstructions of Neanderthals emphasized the similarity rather than differences from modern humans. From the 1940s throughout the 1970s, it was common to use the subspecies classification of Homo sapiens neanderthalensis vs. Homo sapiens sapiens; the hypothesis of "multiregional origin" of modern man was formulated in the 1980s on such grounds, arguing for the presence of an unbroken succession of fossil sites in both Europe and Asia. Hybridization between Neanderthals and Cro Magnon had been suggested on skeletal and craniological grounds since the early 20th century, found increasing support in the 20th century, until Neanderthal admixture was found to be present in modern populations genet
Homo ergaster Homo erectus ergaster or African Homo erectus is an extinct chronospecies of the genus Homo that lived in eastern and southern Africa during the early Pleistocene, between about 1.9 million and 1.4 million years ago. Proposed as a separate species, H. ergaster is now considered either an early form, or an African variety, of H. erectus. The binomial name was published in 1975 by Mazák; the specific epithet, "ergaster", is derived from the Ancient Greek ἐργαστήρ ergastḗr - "workman", in reference to the advanced lithic technology developed by the species, thereby introducing the Acheulean industry. KNM-ER 2598, a "H. erectus-like" occipital bone stands as the earliest evidence for H. erectus in Africa at 1.9 million years ago. There is a fossil gap between 1.9 and 1.6 million years ago, KNM-ER 3733 is the oldest known H. ergaster skull dated to about 1.6 million years ago. Its survival past 1.4 million years ago is uncertain, again due to a fossil gap, the next available African fossils allowing reliable morphological analysis are those of Homo rhodesiensis, at 0.6 million years ago.
South African palaeontologist John T. Robinson discovered in 1949 a mandible of a new hominin in southern Africa, which he named Telanthropus capensis and which today is classified as Homo ergaster; that taxon was first applied to a mandible found near Lake Rudolf, Kenya, by Colin Groves and Vratislav Mazák in 1975. A near-complete skeleton of H. ergaster, KNM-WT 15000, or "Turkana Boy", was discovered in 1984 at Lake Turkana by Kamoya Kimeu and Alan Walker. It is dated to 1.6 million years ago and is one of the most complete early hominin fossils found to date. Although "Homo ergaster" has gained some acceptance as a valid taxon since its proposal in 1975, ergaster and erectus since the 1980s have come to be seen as separate populations of the larger species H. erectus. Some consider H. ergaster to be a variety of H. erectus that migrated out of Africa, branching into various Eurasian subspecies. In this scenario, the labels "Homo erectus sensu stricto" refers to the Eurasian varieties and "Homo erectus sensu lato" for the greater species comprising both Asian and African populations.
The notation "Homo erectus/ergaster" was used to express the uncertainty on whether the two designations should be considered synonyms. The question was described as "famously unresolved" as of 2003. Sura et al concluded that Homo erectus "was a source of multiple events of gene flow to the Eurasian continent"; the discoveries of the Dmanisi skulls in the South Caucasus since 2005 have re-opened this question. Their great morphological diversity suggests that the variability of Eurasian H. erectus includes the African fossils dubbed H. ergaster. The discovery of Dmanisi skull 5 in 2013, dated to 1.8 million years ago, now dates evidence of H. erectus in Eurasia as of the same age as evidence for H. ergaster in Africa, so that it is unclear if the speciation of H. erectus/ergaster from H. habilis took place in Africa or Asia. This has reinforced the trend of considering H. ergaster as synonymous with H. erectus, a species which would have evolved just after 2 million years ago, either in Africa or West Asia, dispersed throughout Africa and Eurasia.
Before the discoveries at Dmanisi there was broad concern about the amount of speciation as a norm described throughout the course of human evolution. A question was raised as to whether the number of taxa within the genus Homo had been overstated. H. Ergaster may be distinguished from H. erectus by its thinner skull-bones and lack of an obvious supraorbital foramen, from H. heidelbergensis by its thinner bones, more protrusive face, lower forehead. Derived features separating it from earlier non-Homo species include reduced sexual dimorphism, a smaller, more orthognathous face, a smaller dental arcade, a larger cranial capacity. Remains have been found in Tanzania, Ethiopia and South Africa. H. erectus/ergaster is the predecessor of Homo heidelbergensis, which arose after about 0.8 million years ago. Late populations of morphological H. erectus were replaced by H. heidelbergensis by 0.5 million years ago. Homo neanderthalensis and Homo sapiens are in turn derived from H. heidelbergensis at about 0.3 million years ago.
There are broad divisions in the scientific community re interpreting the development of the earliest species of genus Homo. H. habilis is accepted as the putative ancestor of Homo, the direct ancestor of H. ergaster. However, habilis's status as a legitimate species within "Homo" is contentious. Habilis and ergaster coexisted in East Africa for half a million years, which indicates that, rather than an anagenetic connection between them, they diverged from a common ancestor; the derivation of H. erectus from Homo habilis is thought to have taken place shortly after two million years ago. By 1.8 million years ago, both African and Asian variants were present. These early descendants of H. habilis may have been discovered at Dmanisi, Georgia as'. H. ergaster remained in Africa for about 500,000 years before disappearing from the fossil record after 1.4 million years ago. The much-later evidence of the derived Homo heidelbergensis i
In taxonomy, Homo sapiens is the only extant human species. The name was introduced in 1758 by Carl Linnaeus. Extinct species of the genus Homo include Homo erectus, extant during 1.9 to 0.4 million years ago, a number of other species. The age of speciation of H. sapiens out of ancestral H. erectus is estimated to have been 350,000 years ago. Sustained archaic admixture is known to have taken place both in Africa and in Eurasia, between about 100,000 and 30,000 years ago; the term anatomically modern humans is used to distinguish H. sapiens having an anatomy consistent with the range of phenotypes seen in contemporary humans from varieties of extinct archaic humans. This is useful for times and regions where anatomically modern and archaic humans co-existed, for example, in Paleolithic Europe; the binomial name Homo sapiens was coined by Linnaeus, 1758. The Latin noun homō means "human being", while the participle sapiēns means "discerning, sensible"; the species was thought to have emerged from a predecessor within the genus Homo around 300,000 to 200,000 years ago.
A problem with the morphological classification of "anatomically modern" was that it would not have included certain extant populations. For this reason, a lineage-based definition of H. sapiens has been suggested, in which H. sapiens would by definition refer to the modern human lineage following the split from the Neanderthal lineage. Such a cladistic definition would extend the age of H. sapiens to over 500,000 years. Extant human populations have been divided into subspecies, but since around the 1980s all extant groups have tended to be subsumed into a single species, H. sapiens, avoiding division into subspecies altogether. Some sources show Neanderthals as a subspecies; the discovered specimens of the H. rhodesiensis species have been classified by some as a subspecies, although it remains more common to treat these last two as separate species within the genus Homo rather than as subspecies within H. sapiens. The subspecies name H. sapiens sapiens is sometimes used informally instead of "modern humans" or "anatomically modern humans".
It has no formal authority associated with it. By the early 2000s, it had become common to use H. s. sapiens for the ancestral population of all contemporary humans, as such it is equivalent to the binomial H. sapiens in the more restrictive sense. The speciation of H. sapiens out of archaic human varieties derived from H. erectus is estimated as having taken place over 350,000 years ago, as the Khoisan split from other populations is dated between 260,000 and 350,000 years ago. An alternative suggestion defines H. sapiens cladistically as including the lineage of modern humans since the split from the lineage of Neanderthals 500,000 to 800,000 years ago. The time of divergence between archaic H. sapiens and ancestors of Neanderthals and Denisovans caused by a genetic bottleneck of the latter was dated at 744,000 years ago, combined with repeated early admixture events and Denisovans diverging from Neanderthals 300 generations after their split from H. sapiens, as calculated by Rogers et al..
The derivation of a comparatively homogeneous single species of H. sapiens from more diverse varieties of archaic humans was debated in terms of two competing models during the 1980s: "recent African origin" postulated the emergence of H. sapiens from a single source population in Africa, which expanded and led to the extinction of all other human varieties, while the "multiregional evolution" model postulated the survival of regional forms of archaic humans converging into the modern human varieties by the mechanism of clinal variation, via genetic drift, gene flow and selection throughout the Pleistocene. Since the 2000s, the availability of data from archaeogenetics and population genetics has led to the emergence of a much more detailed picture, intermediate between the two competing scenarios outlined above: The recent Out-of-Africa expansion accounts for the predominant part of modern human ancestry, while there were significant admixture events with regional archaic humans. Since the 1970s, the Omo remains, dated to some 195,000 years ago, have been taken as the conventional cut-off point for the emergence of "anatomically modern humans".
Since the 2000s, the discovery of older remains with comparable characteristics, the discovery of ongoing hybridization between "modern" and "archaic" populations after the time of the Omo remains, have opened up a renewed debate on the "age of H. sapiens", in journalistic publications cast into terms of "H. sapiens may be older than thought". H. s. idaltu, dated to 160,000 years ago, has been postulated as an extinct subspecies of H. sapiens in 2003. H. Neanderthalensis, which became extinct about 40,000 years ago, has been classified as a subspecies, H. s. neanderthalensis. H. heidelbergensis, dated 600,000 to 300,000 years ago, has long been thought to be a candidate for the last common ancestor of the Neanderthal and modern human lineages. However, genetic evidence from the Sima de los Huesos fossils published in 2016 seems to suggest that H. heidelbergensis in its entirety should be included in the Neanderthal lineage, as "pre-Neanderthal" or "early Neanderthal", while the divergence time between the Neanderthal and
Australopithecus ( OS-trə-lo-PITH-i-kəs. From paleontological and archaeological evidence, the genus Australopithecus evolved in eastern Africa around 4 million years ago before spreading throughout the continent and becoming extinct two million years ago. Australopithecus is not extinct as the Kenyanthropus and Homo genera emerged as sister of a late Australopithecus species such as A. Africanus and/or A. Sediba. During that time, a number of australopithecine species emerged, including Australopithecus afarensis, A. africanus, A. anamensis, A. bahrelghazali, A. deyiremeda, A. garhi, A. sediba. For some hominid species of this time – A. robustus, A. boisei and A. aethiopicus – some debate exists whether they constitute members of the genus Australopithecus. If so, they would be considered'robust australopiths', while the others would be'gracile australopiths'. However, if these more robust species do constitute their own genus, they would be under the genus name Paranthropus, a genus described by Robert Broom when the first discovery was made in 1938, which makes these species P. robustus, P. boisei and P. aethiopicus.
Australopithecus species played a significant part in human evolution, the genus Homo being derived from Australopithecus at some time after three million years ago. In addition, they were the first hominids to possess certain genes, known as the duplicated SRGAP2, which increased the length and ability of neurons in the brain. One of the australopith species evolved into the genus Homo in Africa around two million years ago, modern humans, H. sapiens sapiens. In January 2019, scientists reported that Australopithecus sediba is distinct from, but shares anatomical similarities to, both the older Australopithecus africanus, the younger Homo habilis. Gracile australopiths shared several traits with modern apes and humans, were widespread throughout Eastern and Northern Africa around 3.5 million years ago. The earliest evidence of fundamentally bipedal hominids can be observed at the site of Laetoli in Tanzania; this site contains hominid footprints that are remarkably similar to those of modern humans and have been dated to as old as 3.6 million years.
The footprints have been classified as australopith, as they are the only form of prehuman hominins known to have existed in that region at that time. Australopithecus anamensis, A. afarensis, A. africanus are among the most famous of the extinct hominins. A. africanus was once considered to be ancestral to the genus Homo. However, fossils assigned to the genus Homo have been found. Thus, the genus Homo either split off from the genus Australopithecus at an earlier date, or both developed from a yet unknown common ancestor independently. According to the Chimpanzee Genome Project, the human and chimpanzee lineages diverged from a common ancestor about five to six million years ago, assuming a constant rate of evolution, it is theoretically more for evolution to happen more as opposed to more from the date suggested by a gene clock However, hominins discovered more are somewhat older than the presumed rate of evolution would suggest. Sahelanthropus tchadensis called "Toumai", is about seven million years old and Orrorin tugenensis lived at least six million years ago.
Since little is known of them, they remain controversial among scientists since the molecular clock in humans has determined that humans and chimpanzees had a genetic split at least a million years later. One theory suggests that the human and chimpanzee lineages diverged somewhat at first some populations interbred around one million years after diverging; the brains of most species of Australopithecus were 35% of the size of a modern human brain. Most species of Australopithecus were diminutive and gracile standing 1.2 to 1.4 m tall. In several variations is a considerable degree of sexual dimorphism, males being larger than females. According to one scholar, A. Zihlman, Australopithecus body proportions resemble those of bonobos, leading evolutionary biologists such as Jeremy Griffith to suggest that bonobos may be phenotypically similar to Australopithecus. Furthermore, thermoregulatory models suggest that Australopithecus species were hair covered, more like chimpanzees and bonobos, unlike humans.
Modern humans do not display the same degree of sexual dimorphism as Australopithecus appears to have. In modern populations, males are on average a mere 15% larger than females, while in Australopithecus, males could be up to 50% larger than females. New research suggests, that australopithecines exhibited a lesser degree of sexual dimorphism than these figures suggest, but the issue is not settled. Opinions differ as to whether the species A. aethiopicus, A. boisei, A. robustus should be included within the genus Australopithecus, no current consensus exists as to whether they should be placed in a distinct genus, Paranthropus
Sahelanthropus tchadensis is an extinct species of the Hominini and is the ancestor to Orrorin, dated to about 7 million years ago, during the Miocene epoch very close to the time of the chimpanzee–human divergence. Few specimens other than the partial skull, nicknamed Toumaï, are known. Existing fossils include a small cranium named Toumaï, five pieces of jaw, some teeth, making up a head that has a mixture of derived and primitive features; the braincase, being only 320 cm3 to 380 cm3 in volume, is similar to that of extant chimpanzees and is notably less than the approximate human volume of 1350 cm3. The teeth, brow ridges, facial structure differ markedly from those found in Homo sapiens. Cranial features show a flatter face, u-shaped dental arcade, small canines, an anterior foramen magnum, heavy brow ridges. No postcranial remains have been recovered; the only known skull suffered a large amount of distortion during the time of fossilisation and discovery, as the cranium is dorsoventrally flattened, the right side is depressed.
Sahelanthropus tchadensis may have walked on two legs. However, because no postcranial remains have been discovered, it is not known definitively whether Sahelanthropus was indeed bipedal, although claims for an anteriorly placed foramen magnum suggests that this may have been the case. Upon examination of the foramen magnum in the primary study, the lead author speculated that a bipedal gait "would not be unreasonable" based on basicranial morphology similar to more recent hominins; some palaeontologists have disputed this interpretation, stating that the basicranium, as well as dentition and facial features, do not represent adaptations unique to the hominin clade, nor indicative of bipedalism. Further, according to recent information, what might be a femur of a hominid was discovered near the cranium—but which has not been published nor accounted for. Fifteen years after the discovery of the fossil, the anthropologist Roberto Macchiarelli—professor at the University of Poitiers and the Museum of Natural History of Paris—suspects Michel Brunet and his laboratory in Poitiers of blocking information about a femur found close to the skull.
That the laboratory would have delayed identification may question the bipedalism of Toumaï. The fossils were discovered in the Djurab Desert of Chad by a team of four led by a Frenchman, Alain Beauvilain, three Chadians, Adoum Mahamat, Djimdoumalbaye Ahounta, Gongdibé Fanoné, members of the Mission paleoanthropologique Franco-tchadienne led by Michel Brunet. All known material of Sahelanthropus was found between July 2001 and March 2002 at three sites: TM 247, TM 266, which yielded most of the material, including a cranium and a femur, TM 292; the discoverers claimed that S. tchadensis is the oldest-known human ancestor after the split of the human line from that of chimpanzees. The bones were found far from most previous hominin fossil finds, which are from Eastern and Southern Africa. However, an Australopithecus bahrelghazali mandible was found in Chad by Mamelbaye Tomalta and Alain Beauvilain, Michel Brunet and Aladji H. E. Moutaye as early as 1995. With the sexual dimorphism known to have existed in early hominins, the difference between Ardipithecus and Sahelanthropus may not be large enough to warrant a separate species for the latter.
Sahelanthropus may represent a common ancestor of humans and chimpanzees, though no consensus has been reached yet by the scientific community. The original placement of this species as a human ancestor but not a chimpanzee ancestor would complicate the picture of human phylogeny. In particular, if Toumaï is indeed a direct human ancestor its facial features bring into doubt the status of Australopithecus whose thickened brow ridges were reported to be similar to those of some fossil hominins, where the brow ridge morphology of Sahelanthropus differs from that observed in all australopithecines, most fossil hominins and extant humans. Another possibility is that Toumaï is related to both humans and chimpanzees, but is the ancestor of neither. Brigitte Senut and Martin Pickford, the discoverers of Orrorin tugenensis, suggested that the features of S. tchadensis are consistent with a female proto-gorilla. If this claim is upheld the find would lose none of its significance, because at present few chimpanzee or gorilla ancestors have been found anywhere in Africa.
Thus if S. tchadensis is an ancestral relative of the chimpanzees or gorillas it represents the earliest known member of their lineage. And S. tchadensis does indicate that the last common ancestor of humans and chimpanzees is unlikely to resemble extant chimpanzees, as had been supposed by some paleontologists. A further possibility, highlighted by research published in 2012, is that the human–chimpanzee split is earlier than thought, with a possible range of 7 to 13 million years ago, based on slower than thought changes between generations in human DNA. Indeed, some researchers consider suggestions that Sahelanthropus is too early to be a human ancestor to have evaporated. Sediment isotope analysis of cosmogenic atoms in the fossil yielded an age of about 7 million years. In this case, the fossils were found exposed in loose sand. In fact, Toumaï may have been reburied in the r