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Archive for July, 2013

Our Human Origins—Part II

Evolution is the Real Story

 

The Evidence for Evolution Goes Into High Gear

Raymond Dart was one of the most significant figures in the history of human origins and evolution. In 1925, he reported in the journal Nature one of the most important findings in all of Paleoanthropology. He discovered the fossil skull of a child, half-ape, half-human. It was the first evidence of an early fossil link between apes and man. This became known as the now famous Taung child, named after the location where it was found. The location was in southern Africa.[1]

The Taung baby was a three year old Australopithecus africanus. This fossil had been found in a limestone quarry near Taung in Botswana and sent to Raymond Dart, at the University of Witwatersrand in 1924. He immediately recognized its great importance.

According to Richard E. Leakey in Origins, “Dart discovered  that the skull was relatively large compared with other known non-human primates; the teeth were more hominid-like than ape-like, as was the shape of the face. Judging by the estimated angle at which the head joined with the neck, the creature stood and moved in an upright posture, and he declared so in the scientific journal, Nature, at the beginning of 1925.”[2]

He named his Taung child in scientific nomenclature as Australopithecus africanus, the southern ape of Africa.

Dart described what he saw in great detail following the canons of a good researcher:

“Dart wasted no time in preparing the report for submission to Nature, in essence, it pointed out this while the skull, teeth, and jaw of the child had been humanoid, rather than anthropoid or apelike, this was undoubtedly a small-brained hominid, or member of the human family – the first of its kind to be described. He pointed out that the forward position of the foramen magnum, where the spinal cord attached to the skull, clearly indicated that this hominid had walked upright, with its hands free for the manipulation of tools and weapons in an open environment far to the south of the equatorial forests inhabited by chimpanzees and gorillas.”[3]

It’s fair to say that Dart’s findings were not immediately accepted by the scientific community. There were primarily two major objections to his Taung child discovery; one related to the Taung child itself, and the other to its location. The first was that juvenile apes tend to look more human-like than adult apes. This caused many to think that he had not found the missing link between man and ape. The second criticism was a kind of scientific prejudice in that it required further evidence before it was resolved. That is, the scientific community wanted to believe that the cradle of man’s origins must be in Europe or Asia, not Africa. Many years later the scientific community would accept and embrace the fossil findings of Raymond Dart.

The Leakey Family

One of the most famous scientists was part of a family of prolific productive scientists known as the Leakey’s. They comprised the famous Louis Leakey, Mary Leakey, Richard Leakey and Meave Leakey.

Louis Leakey

Louis Leakey had a tremendous impact on the study of human origins. Besides writing 20 books and 150 articles he found multiple fossils and stone tools. Due to his efforts he convinced other scientists that Africa was the key location in which to search for human origins. In essence he thought Africa was the cradle of humanity.[4]

Louis Semour Bazett Leakey was born on August 7, 1903 at Kabete Mission (near Nairobi Kenya). He was the son of English missionaries to the Kikuyu tribe. He graduated in 1926 with both degrees in archaeology and anthropology. After this, Leakey began leading expeditions to Olduvai, a river gorge in Tanzania where he found important fossils and Stone Age tools. In 1948 he reported finding a 20-million year old skull, which he named Proconsul africanus.[5]

The first hominid fossil attributed to Leakey was a robust skull with huge teeth dated to 1.75 million years ago. It was found by Leakey’s collaborators and second wife, Mary Douglas Leakey. It was found in deposits that also contained stone tools. He called the find Zinjanthropus boisei (it is now considered to be a form of Australopithecus). In 1964 Louis announced another important discovery–that of Homo habilis which Leakey believed was the first member of the actual human genus as well as the first toolmaker.[6]

Leakey brought world attention to his findings. But he also was influential in the emerging field of primatology. He was responsible for initiating Jane Goodall’s long-term field study of chimpanzees in the wild, and he helped obtain and coordinate funding for similar projects such as Dian Fossey’s work with mountain gorillas in Rwanda, and Birute Galdikas-Brindemour’s work with orangutans in the Sarawak region of Indonesia. Because of all he discovered and all his contributions to scientific research on human origins, Louis Leakey can certainly be called the “Michael Jordon” of human origins research. On route to a speaking engagement in London in 1972 he suffered a heart attack and died.[7]

Mary Leakey

Mary Leakey was Louis Leakey’s wife and collaborator, who made very important contributions to the field of paleoanthropology and research on human origins. Mary Douglas Nicol Leakey was born on February 6, 1913 in London, England.[8] She had a difficult childhood and when her father died in 1926, Mary was sent to a Catholic convent where she was repeatedly expelled. Her life soon changed for the better. After she saw the caves of the Dordogne she resolved to earn a degree in prehistory. She attended lectures at the University of London concerning archaeology and geology.

Mary met Louis when she was asked to illustrate a book, The Desert Fayoum written by Dr. Canton-Thompson.[9] Dr. Canton-Thompson acted as matchmaker to arrange for Mary to meet Louis Leakey while he was giving a talk at the Royal Anthropological Institute. Louis asked Mary to illustrate his book, Adam’s Ancestors.[10]  Their relationship grew thereafter.

“Mary and Louis spent from 1935 to 1959 at Olduvai Gorge in the Serengeti Plains of northern Tanzania where they worked to reconstruct many Stone Age cultures dating as far back as 100,000 years to two-million years ago. They documented stone tools from primitive stone-chopping instruments to multi-purpose hand axes.”[11]  “In 1947, on Rusinger Island Mary unearthed a Proconsul africanus skull which was the first skull of a fossil ape ever to be found. It was dated to be 20 million years old…In 1955 Mary and Louis was jointly awarded the Stopes Medal from the Geological Association for their hard work and discoveries.”[12]

In 1959 a 1.75 million year old Australopithecus boisei skull was found.[13]Not long afterwards, a less robust Homo habilis skull and bones of a hand were found. Both fossils were believed to be that of stone-tool peoples.[14] In 1965 Mary and Louis uncovered a Homo erectus cranium which was one million years old. After Louis Leakey died in 1972, Mary continued her work at Olduvai and Laetoli. She discovered Homo fossils at Laetoli which were found to be more than 3.75 million years old, fifteen species and one new genus.[15]

One famous find of Mary Leakey occurred between 1978 and 1981. This was the Laetoli hominid footprint trail which was left in volcanic ashes 3.6 million years ago.[16] The Laetoli footprints were significant. Laetoli is in Tanzania, which is located in eastern Africa. Mary started excavating the site at Laetoli in 1974. The work on the site continued for five years, when in 1978, she found three sets of fossilized footprints preserved in the ground.[17]

There were approximately seventy footprints found in two parallel trails about thirty meters long. After studying the footprints, Mary came to the conclusion that the prints were made by Australopithecus afarensis that had been walking bipedally. This raised questions about the evolution of bipedalism. Prior to this, it was believed that the first hominid to walk bipedally was Homo erectus.[18]

Although Mary Leakey was technically an archaeologist her work was mostly as a physical anthropologist. She was mostly known for the 1959 excavation of a two-million-year-old fossilized human skull. She also helped the world understand that the evolution of humans follows a principle rather than a theory. Mary Leakey died in 1996 at the age of 83.[19]

 

Richard Leakey

Richard Erskine Frere Leakey is the second of three sons of archaeologists Louis and Mary Leakey. He was born on December 19, 1944. Richard Leakey has the triple distinction of being a paleontologist, archaeologist, and conservationist. Richard Leakey started his career following in the footsteps of his famous parents with discoveries of early hominid fossils in East Africa. A Homo habilis skull (ER 1470) and a Homo erectus skull (ER 3733) were discovered respectively in 1972 and 1975.[20] These were among his most significant finds of Leakey’s earlier foundations.

In 1969, he and his wife Margaret had a daughter, Anna, and they were divorced in the same year. The following year he married Meave Epps, a zoologist who specialized in primates. They have two daughters, Louise in 1972, and Samira in 1974.

In the 1970s Richard’s fossil hunting continued. In 1984 his team found the most impressive fossil of his career. WT 15000, nicknamed The Turkana Boy, was a nearly complete skeleton of a Homo erectus boy.[21] The following year he made another major find, WT 17000, the first skull of the Australopithecus aethiopicus.[22] Richard Leakey and Roger Lewin described the experience of this find and their interpretation of it, in their book Origins Reconsidered (1992). Turkana Boy was estimated to be 1.6 million years old.[23]

In 1989 Richard Leakey was appointed the head of the Kenyan Wildlife Service (KWS) by President  Daniel Arap Moi in response to the international outcry over the poaching of elephants and the impact it was having on the wildlife of Kenya. With his appointment Leakey took steps to create well-armed anti-poaching units that were authorized to shoot poachers on sight. As a result the poaching menace was dramatically reduced. In 1989 Richard Leakey, along with President Arap Moi and the KWS made the international news headlines when a stockpile of 12 tons of ivory was burned.[24] In May 1995 Richard joined a group of Kenyan intellectuals in launching a new political party–the Safina Party.[25]

Two years earlier in 1993 Richard Leakey lost both his legs when his propeller-driven plane crashed. Sabotage was suspected, but never proven. In a few months Richard Leakey was walking again on artificial limbs. Richard Leakey wrote about his experiences with the KWS in his book Wildlife Wars: My Battle to Save Kenya’s Elephants (2001).[26] In recent years Richard, although interested in the field, has had little to do with paleontology. Most of his time was devoted to combating elephant and rhino poaching and overhauling Kenya’s troubled park system. Richard’s wife Meave Leakey continues to produce research findings in the field of paleontology.

Meave Leakey

Meave (Epps) Leakey was born in LondonEngland in 1942.[27] She was educated in convent and boarding schools. She later attended a technical college and university, where she developed a love of science. She obtained a B.S. (in Zoology and Marine Zoology) and Ph.D. (Zoology) from the University of North Wales.[28] In 1970, Meave and Richard Leakey were married. As mentioned earlier, they have two children: Louise in born 1972, and Samira, born in 1974. In addition to her fieldwork at Turkana, Meave Leakey’s research focused on the evolution of East African fossil mammals and mammalian faunas as documented in the Turkana basin.

Dr. Meave Leakey is the standard-bearer of a family of paleoanthropologists who have dominated their field for most of the 20th Century. In 1999 Dr. Meave Leakey made a discovery at Lake Turkana that completely redefined our understanding of early human ancestry. The research team found a 3.5 million-year-old skull and partial jaw (which she named Kenyanthropus platyops, or flat-faced man of Kenya.[29] They considered it to belong to a new branch of our early human family. This finding challenged the view that human beings descended from a single line of evolution.[30]

In 1989, Meave Leakey became the coordinator of the National Museum of Kenya’s paleontological field research in the Turkana Basin. Since her appointment Dr. Leakey has focused at the Turkana site in finding evidence of the very earliest human ancestors, concentrating on sites between 8 and 4 million years ago. In 1994, remains of some of the earliest hominids known were discovered at Kanapoi, southwest of the lake region. Not only do these finds represent a new species of Australopithecus anamensis (a likely ancestor of Australopithecus afarensis, the earliest hominid species previously recognized), but the dating of the find at 4 million years old has called for revising the accepted timeline for the evolution of hominids.[31] Dr. Meave Leakey has written more than fifty scientific articles and books.

Other Scientists

Donald C. Johanson

Dr. Donald C. Johanson, who earned his Ph.D. from the University of Chicago in 1974, is a sort of celebrity in the field of paleontology. He is currently professor of Anthropology and Director of the Institute of Human Origins at Arizona State University. He is best known for his discovery of “Lucy,” a 3.2 million-year-old Australopithecus afarensis skeleton he found in Ethiopia in 1974.[32] His books include Lucy: The Beginnings of Humankind and From Lucy to Language. Dr. Johanson hosted the Emmy-nominated Nova television series In Search of Human Origins. Johanson’s career, which spans more than 30 years, has led him to undertake field explorations in Ethiopia, Tanzania, Egypt, Jordon, Saudi Arabia, Yemen, Entrea, and most recently Iran.[33]

Johanson’s book, Lucy: The Beginnings of Humankind was winner of the 1981 American Book Award in Science. It chronicled his discovery of the 3.2 million-year-old Lucy skeleton and highlights its importance for comprehending who we are and where we come from.[34]

The expedition to find Lucy began in 1974. The findings took place in the badlands of the Afar Triangle, Middle Awash area near Hadar, Ethiopia in northeast Africa. During the Pliocene this area was dominated by lakes and woodlands. It was along the edge of these now-dried lakes among the former woodland that fossils can be located. Lucy was a 40%–complete skeleton. In an area known as AC-288-1 (Afar Locality # 288) Lucy was found in a sedimentary layer that eventually was dated at 3.5 million-years-ago.[35]

“Johanson, along with colleague Tom Gray, had been mapping another locality at the Afar site. Feeling ‘lucky,’ Johanson took a short detour into another area later mapped as locality 288 and ‘noticed something lying on the ground partway up the slope’ (Johanson, Edey, 1980). This ‘something’ turned out to be the exposed portion of a hominid arm bone.”[36]

Later Johanson and his team sectioned off the site to prepare for the collecting of the remaining bones. After three weeks of work, they had collected several hundred pieces of bone, which represented 40% of a single skeleton. The team knew these bones belonged to a single individual because there was no duplication of any one bone. During the night of November 30, 1974 there was much celebration and excitement over the discovery. There was drinking, dancing, and singing. The Beatles’ song ‘Lucy in the Sky with Diamonds’ was playing over and over.  The suggestion was made by Johanson that maybe they ought to call their find–Lucy.[37]

Dr. Johanson has asserted that Lucy was bipedal based on the shape of her pelvis and the angle the femur takes from the hip socket to the knee joint. “From her waist down she was hominid, and from her waist up she was still ape, as her skull was still the size of a chimpanzee. The discovery of Lucy proves that bipedalism predates big brain size.”[38]

What do we know about Lucy?

  • Other Australopithecus afarensis bones addressing the issue of sexual dimorphism suggest Lucy was female.
  • Lucy’s pelvic and femur structures, along with her knee joint confirms that she was decidedly hominid.
  • Lucy’s arms are also shorter than that of an ape, while maintaining a longer ratio of arm length to overall height than that of a modern human.
  • From her skeleton, comparing it to chimps and humans, and seeing that she is somewhere in between, she is on the way to becoming human.
  • Lucy’s dentition is a cross between ape and human in that the overall shape is apelike while the canine tooth size resembles that of modern humans.
  • In a chimp’s mandible there is a space between its incisors and large canine, which does not occur in Australopithecus afarensis females.
  • Lucy’s jaw was not as pronounced as that of the ape.
  • In addition to having characteristics halfway between ape and hominid, Lucy proves that bipedalism was prevalent long before large brain size.[39]

“It is generally accepted that hominids diverged from the apes between 6 and 5 million years ago. Australopithecus afarensis led to 2 further divergences, the robust Australopithecines that died out, and the gracile line leading eventually to us.”[40]

Recent Developments in Evolution Research

In 2005 some segments of American Society fought to suppress and dilute the teaching of even the most basic facts of evolution.  With this in mind Science Magazine decided to put Darwin in the spotlight by selecting several discoveries, each of which reveals the laws of evolution in action.

On December 23, 2005 Elizabeth Culotta and Elizabeth Pennisi wrote an article in Science Magazine.[41] The name of the article was: Breakthrough of the Year: Evolution in Action. In it the two authors describe the most significant studies and findings on evolution during 2005. “Equipped by genome data and field observations of organisms from microbes to mammals, biologists made huge strides toward understanding the mechanisms by which living creatures evolve.”[42]

Two areas of interest in the article were:

The Chimpanzee Genome

Speciation

 

The Chimpanzee Genome

In September 2005 a dramatic result occurred when an international team published the genome of our closest relative, the chimpanzee. With the human genome defined, researchers could begin to line up chimp and human DNA and examine, one by one, the 40 million evolutionary events that separate them from us. The genome data confirmed our close kinship with chimps.[43]

“We differ by only about 1% in the nucleotide bases that can be aligned between the two species, and the average protein differs by less than two amino acids. But a surprisingly large chunk of noncoding material is either inserted or deleted in the chimp as compared to the human, bringing the total difference in DNA between our two species to about 4%.”[44]

Speciation

The year 2005 was a standout for those researchers studying the emergence of new species, or speciation. As researchers know, a new species can form when populations of an existing species begin to adapt in different ways and eventually stop interbreeding.

This often occurs when populations are separated by geographical barriers like oceans or mountain ranges. But sometimes a single, continuous population simply splits in two. The prediction would be that the splitting begins when some individuals in a population stop mating with others. During 2005 field biologists recorded compelling examples of that process, often which results in rapid evolution of the organisms’ shape and behavior.[45]

The authors use the example of birds called European blackcaps sharing breeding grounds in southern Germany and Austria. Evidently these warblers migrate to northerly grounds in the winter rather than heading south. Isotopic data revealed that northerly migrants reach the common breeding grounds earlier and mate with one another before southerly migrants arrive. The upshot of this is that differences in timing may drive the two populations to become two species.[46]

Other Findings

Some of the most exciting developments in paleontology took place 2003-2005.

In June 2003 three new skulls from Herto, Ethiopia were discovered. These are the oldest known modern human fossils, at 160,000 years old.[47] The discoveries assigned them to a new species, Homo sapiens idalto. They claim they are anatomically and chronologically intermediate between older archaic humans and the more recent fully modern humans. Their age and anatomy is cited as strong evidence for the emergence of modern humans from Africa, and against the multiregional theory which argues that modern humans evolved in many places around the world.[48]

In March 2004 some fragmentary fossils discovered in Ethiopia were dated between 5.2 and 5.8 million years ago were originally assigned to a new subspecies, Ardipithecus ramidus kadabba. Following further study, the finders have decided that the differences between them and other fossils justify assigning them to a new species, Ardipithecus kadabba.[49] Also in March 2004 it was reported that the details of four new mtDNA sequences were retrieved from Neanderthal fossils. “This brings the number of known Neanderthal mtDNA sequences to eight, all of which are closely related, and considerably different from all modern human mtDNA sequences.”[50]

Probably the most significant finding in 2004 was the discovery of a gene by scientists at the University of Pennsylvania and the Children’s Hospital of Pennsylvania. It is the first scientific report that correlates human genetics directly with the fossil record of our hominid ancestors. They theorized that a jaw gene called key to evolution made room for a larger brain.[51]

Basically, a tiny genetic change in the muscles of pre-humans millions of years ago may have played a major role in endowing modern Homo sapiens with the larger brain and the capacity for thought, language and tool-making that distinguishes us from apes.

The novel theory was advanced by a team of biologists and surgeons, suggesting that a mutation in a single gene some 2.4 million years ago was largely responsible for a crucial change in the shape of our ancestors’ jaws and allowed for skulls with room for brains far larger than earlier members of the hominid line.[52]

The mutated gene is one of a class of codes for a protein called myosin, which is responsible for muscle contraction and determines the strength and size of the chewing and biting muscles of the jaws. In modern humans the mutated myosin gene, known as MYH16 – differs from the far older un-mutated gene found in many nonhuman primates, including the macaques and chimpanzees.[53]

The researchers contend that the mutated gene in effect disabled the large and powerful jaw muscles found in fossils of earlier large-skulled hominids and thereby launched a lineage of pre-humans with smaller jaws and larger skulls with plenty of room for bigger brains. This occurred 2.4 million years ago–just about the time when our own lineage split off from more primitive hominids known as the Australopithecines.[54]

Two skulls in 1967 were found near the Omo River in Ethiopia by Richard Leakey and thought to be 130,000 years old. However, it has been dated at 195,000 years, the oldest date known for a modern human skull. The Omo I skull is fully modern, while Omo II has some archaic features.[55]

In March 2005 a newly discovered partial skeleton from Mille in Ethiopia is claimed to be the world’s oldest bipedal hominid. “The fossil is about 4 million years old and has not yet been classified or published in the scientific literature, though it is said to fall between Ardipithecus ramidus and Australopithecus afarensis”.[56]

     In 2006, Tim White, now a professor at the University of California, Berkeley, announced the discovery of bones from at least 8 Australopithecus anamensis individuals dating to 4.1 million years ago in what had been a woodland environment in the Awash Valley of Ethiopia.  That’s more than a million years before Lucy, who was previously thought to be the earliest hominid skeleton. According to White, his team’s discovery is the “closest we have ever been able to come” to finding the missing-link common ancestor between humans and chimps.[57]

     In 2009, White and his team of researchers announced the discovery of a 4.4 million year old ape/transitional species named Ardipithecus ramidus that also lived in woodland environments of the Awash Valley.  White believes that this very early species was the direct ancestors of Australopithecus afarensis.[58]

     In 2010, Lee Berger of the University of the Witwatersrand in Johannesburg, South Africa announced his discovery of two partial skeletons of what may be a new australopithecine species that lived 1.977 million years ago in South Africa.  He named it Australopithecus sediba (“sediba” means “fountain” or “wellspring” in the Sesotho language of South Africa).  Berger and his colleagues suggest that this new species may be descended from Australopithecus africanus and could be one of the last links in the evolutionary line between the australopithecines and our genus Homo.[59]

In Part III of this series next month I will discuss the current relevance of our human origins and how each one of us can find out his own journey since humans began leaving Africa between 160,000-200,000 years ago.

 


[1] C.K. Brain, “Raymond Dart and Our African Origins,” [online]; accessed 18 Feb. 2005; available from http://www.press.uchicago.edu/misc/Chicago/284158-brain.html.

[2] Origins, 93,95

[3] C.K. Brain

[4] “The Leakey Foundation – Louis S. B. Leakey”, [online]; as accessed 31 March 2005 http://www.leakeyfoundation.org/foundation/fl-2.jsp.

[5] Ibid.

[6] Ibid.

[7] Ibid.

[8] “Mary Leakey,” [online]; accessed 1 Apr. 2005; available from  http://www.musu.edu/emuseum/information/biography/klmno/leakey-mary.html.

[9] Ibid.

[10] Ibid.

[11] Ibid.

[12] Ibid.

[13] Ibid.

[14] Ibid.

[15] Ibid.

[16] Ibid.

[17]Laetoli,” [online]; accessed 1 Apr. 2005;available from   http://www.mnsu.edu/emuseum/archaeology/sites/africa/laetoli.html.

[18] Ibid.

[19] Mary Leakey

[20] “Richard Leakey,” Wikipedia, the free encyclopedia [online]; accessed 25 Jan. 2006; available from, http://en.wikipedia.org/wiki/Richard-Leakey.

[21] “Biographies: Richard Leakey,” [online]; accessed 25 Jan. 2006;available from  http://www.talkorigins.org/faqs/hums/rLeakey.html.

[22] Ibid.

[23] Richard Leakey

[24] Ibid.

[25] Ibid.

[26] Ibid.

[27] “The Leakey Foundation – Meave Leakey,” [online]; accessed 25 Jan. 2006; available from  http://www.leakeyfoundation.org/foundation/fl-6.jsp.

[28] Ibid.

[29] Ibid.

[30] Ibid.

[31] Ibid.

[32]“Donald Johanson Web Page,” [online];accessed 25 Jan. 2006;available from  http://www.asu.edu/clos/iho/johanson.html.

[33] Ibid.

[34] Ibid.

[35] “Australopithecus afarensis (Lucy),” [online];accessed 27 Jan. 2006;available from  http://www.anthro4n6.net/lucy/.

[36] Ibid.

[37] Ibid.

[38] Ibid.

[39] Ibid.

[40] Ibid.

[41] Elizabeth Cullotta and Elizabeth Pennisi, “Breakthrough of the Year: Evolution in Action,” [online] (Science   23 December 2005: Vol 310. no.5756, pp 1878-1879;accessed 29 Dec. 2005;available from  http://www.sciencemap.org/cgi/content/full/310/5756/1878.

[42] Ibid.

[43] Ibid.

[44] Ibid.

[45] Ibid.

[46] Ibid.

[47] “Recent Developments in Paleoanthropology,” [online]; accessed 27 Jan. 2006;available from http://www.talkorigins.com/faqs/homs/recent.html.

[48] Ibid.

[49] Ibid.

[50] Ibid.

[51] David Perlman, Chronicle Science Editor, “Jaw gene mutation called key to evolution. It made room for larger brain researchers theorize,” [online]; accessed 17 May. 2005; available from http://www.sfgate.com/cgi-bin/article.cgi?file=/chronicle/archive/2004/03/25/MUTATION.T.

[52] Ibid.

[53] Ibid.

[54] Ibid.

[55] Recent Developments in Paleoanthropology

[56] Ibid.

[57] ime/specials/packages/article/0,28804,1972075_1972078_1972462,00.html#ixzz2XxFFPk20

[58] Ibid.

[59]f Berger, L. R.; de Ruiter, D. J.; Churchill, S. E.; Schmid, P.; Carlson, K. J.; Dirks, P. H. G. M.; Kibii, J. M. (2010). “Australopithecus sediba: a new species of Homo-like australopith from South Africa”. Science 328 (5975): 195–204. doi:10.1126/science.1184944. PMID 20378811.

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