Human evolution

• 1. Introduction

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  Human evolution did not move in a straight line. The source describes it as weblike, because modern humans interbred with archaic humans rather than descending in a single clean chain. Homo sapiens is one distinct species inside the hominid family of primates, the family that holds all the great apes. Over a long evolutionary history, humans gradually picked up traits like bipedalism, dexterity, and complex language. That word sapiens is Latin for wise or intelligent, and anatomically modern humans emerged in Africa roughly 300,000 years ago. Studying where humans came from pulls together physical and evolutionary anthropology, paleontology, and genetics. The field carries other names too, including anthropogeny, anthropogenesis, and anthropogony. What follows asks how a tree-dwelling primate lineage produced an upright, big-brained species. It asks why our DNA is about 98.4% identical to a chimpanzee's, yet our story includes Neanderthals, Denisovans, and a so-called hobbit on an Indonesian island. And it asks how scientists pieced this together from scattered bone, a song played in a desert camp, and the strength of a chemical reaction in blood.

• 2. Branching From The Apes

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  Primates diverged from other mammals in the Late Cretaceous period, with their earliest fossils appearing over 55 million years ago during the Paleocene. The evolutionary history of primates can be traced back roughly 65 million years, though scarce fossils leave the early details largely unknown. One of the oldest primate-like mammals, Plesiadapis, came from North America, while another, Archicebus, came from China. Early forms such as Altiatlasius and Algeripithecus turned up in Northern Africa. The Faiyum depression southwest of Cairo preserves a surviving tropical population that gave rise to all living primates, from the lemurs of Madagascar to the great apes. Kamoyapithecus, found at Eragaleit in the northern Great Rift Valley in Kenya and dated to 24 million years ago, is the earliest known catarrhine. In 2010, Saadanius was described as a close relative of the last common ancestor of the crown catarrhines, tentatively dated to 29 to 28 million years ago and helping fill an 11-million-year gap in the record. Molecular evidence places the gibbon line splitting from the great apes some 18 to 12 million years ago, with orangutans diverging at about 12 million years. Fossil proto-orangutans may be represented by Sivapithecus from India and Griphopithecus from Turkey, dated to around 10 million years ago.

• 3. The Hominin Split

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  Nakalipithecus fossils found in Kenya may represent a species close to the last common ancestor of gorillas, chimpanzees, and humans. Molecular evidence suggests that between 8 and 4 million years ago, first the gorillas and then the chimpanzees of the genus Pan split off from the line leading to humans. The fossil record for both is thin, partly because rain forest soils tend to be acidic and dissolve bone. The earliest fossils argued by some to belong to the human lineage are Sahelanthropus tchadensis at 7 million years and Orrorin tugenensis at 6 million years, followed by Ardipithecus between 5.5 and 4.4 million years ago. A study of Ardipithecus ramidus found affinities between its skull and those of infant and juvenile chimpanzees, suggesting a paedomorphic craniofacial form. The same work argued that very early hominins, like the less aggressive bonobo, may have evolved through a process of self-domestication. The authors challenged the so-called chimpanzee referential model, writing that Ar. ramidus shares with bonobos reduced sexual dimorphism. They proposed that many basic human adaptations evolved in the ancient forest and woodland of late Miocene and early Pliocene Africa. If they are right, the behavior of chimpanzees may have evolved after the split, rather than preserving our ancestral state.

• 4. Australopithecus And Lucy

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  The genus Australopithecus evolved in eastern Africa around 4 million years ago before spreading across the continent and becoming extinct about 2 million years ago. Its species include Australopithecus anamensis, A. afarensis, A. africanus, and A. sediba, with debate over whether robust forms like P. robustus and P. boisei belong in a separate genus, Paranthropus. Australopithecus prometheus, known as Little Foot, has been dated at 3.67 million years old through a new dating technique. Given the opposable big toe found on Little Foot, the specimen was clearly a good climber, and it may have built a nesting platform in the trees at night, much as chimpanzees and gorillas do. In 1925, Raymond Dart described Australopithecus africanus from the Taung Child, an australopithecine infant found in a cave. The brain was small at 410 cubic centimeters, but rounded and more like a modern human's than a chimpanzee's, and the position of the foramen magnum pointed to bipedal locomotion. Decades later, in 1974, Donald Johanson found Lucy near Hadar in the Afar Triangle of northern Ethiopia. Lucy is the most complete fossil member of Australopithecus afarensis, with a small brain but pelvis and leg bones nearly identical in function to a modern human's. The nickname came from the Beatles song Lucy in the Sky with Diamonds, played loudly and repeatedly in the camp during the excavations.

• 5. The Rise Of Homo

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  The Ledi jaw, dated between 2.75 and 2.8 million years ago, is the earliest documented representative of the genus Homo and arguably the earliest with positive evidence of stone tool use. The brains of these early hominins were about the size of a chimpanzee's, though this may be when the human SRGAP2 gene doubled, speeding the wiring of the frontal cortex. Homo habilis lived from about 2.8 to 1.4 million years ago, diverging from the australopithecines with smaller molars and larger brains. Louis Leakey gave it the name homo habilis, Latin for handy man, because it made tools from stone and perhaps animal bones. With Homo erectus and Homo ergaster, cranial capacity doubled to 850 cubic centimeters during a period of rapid encephalization. Such growth is equivalent to each generation having 125,000 more neurons than their parents. The first fossils of Homo erectus were discovered by Dutch physician Eugene Dubois in 1891 on the Indonesian island of Java. He first called the material Anthropopithecus erectus, then Pithecanthropus erectus, viewing it as intermediate between humans and apes. The German paleoanthropologist Franz Weidenreich later compared Dubois's Java Man with Peking Man and, in 1940, concluded both belonged in a single species, Homo erectus. Richard Wrangham argues that control of fire and cooking released nutritional value and separated Homo from the tree-sleeping australopithecines.

• 6. Cousins Across The Map

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  Homo heidelbergensis lived from about 800,000 to about 300,000 years ago, and its story begins with the Mauer mandible, a primitive fossilized jaw found by miners in Germany in 1907. Otto Schoetensack analyzed the jaw and named the new species, which a 2009 study concluded was most likely an Afro-European common ancestor of both Neanderthals and Homo sapiens. Homo floresiensis, nicknamed the hobbit for its small size, lived from approximately 190,000 to 50,000 years before present, possibly through insular dwarfism. The main find, discovered in 2003, was a skeleton of a woman about 30 years old, estimated at one meter tall with a brain volume of just 380 cubic centimeters, less than a third of the modern human average of 1400. In 2016, smaller ancestral fossils dating to about 700,000 years ago were found at Mata Menge, about 74 kilometers from Liang Bua. Homo neanderthalensis lived in Europe and Asia from 400,000 to about 28,000 years ago, with a surface to volume ratio even lower than modern Inuit populations and a brain larger than that of modern humans. Endocranial estimates suggest Neanderthal groups may have been limited to 120 individuals, compared to 144 possible relationships for modern humans. In 2008, a finger bone of a juvenile Denisovan was uncovered at Denisova Cave in the Altai Mountains of Siberia, with cave artifacts carbon dated to around 40,000 years before present. A small number of specimens from the island of Luzon, dated 50,000 to 67,000 years ago, were assigned to yet another species, Homo luzonensis.

• 7. The Weave Of Interbreeding

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  A 40,000-year-old human skeleton from Romania carried a genome that was 11% Neanderthal, implying a Neanderthal ancestor only four to six generations earlier. All modern non-African humans carry about 1% to 4% of their DNA from Neanderthals, and Neanderthals and Denisovans together may have contributed up to 6% of the genome to some present-day people. A 2010 sequencing of the Neanderthal genome showed interbreeding with anatomically modern humans around 45,000 to 80,000 years ago, near the time modern humans migrated out of Africa. Sergi Castellano of the Max Planck Institute for Evolutionary Anthropology reported in 2016 that Siberian Neanderthal genomes show more similarity to modern human genes than European ones, suggesting a separate interbreeding around 100,000 years ago in the Near East. Human leukocyte antigen haplotypes from Denisovans and Neanderthals make up more than half the HLA alleles of modern Eurasians, evidence of strong positive selection. Corinne Simoneti and her team at Vanderbilt University in Nashville found, from medical records of 28,000 people of European descent, that Neanderthal DNA segments may be associated with a higher rate of depression. As much as 6% of the DNA of some modern Melanesians derives from Denisovans, pointing to limited interbreeding in Southeast Asia. Anatomically modern Homo sapiens and Neanderthals may have co-existed in Europe for as long as 10,000 years, with modern populations exploding and possibly outcompeting Neanderthals by sheer numbers.

• 8. The Upright Body

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  Bipedalism is the basic adaptation of the hominid and the main cause behind a suite of skeletal changes shared by all bipedal hominids. The earliest bipeds, Sahelanthropus or Orrorin, arose some 6 to 7 million years ago, with Ardipithecus a full biped by approximately 5.6 million years ago. Walking on two legs may have freed the hands for carrying food, saved energy, enabled long-distance running, widened the field of vision, and reduced surface area exposed to direct sun. The most significant changes occurred in the pelvic region, where the iliac blade shortened and widened to keep the center of gravity stable. That bowl-like pelvis came with a smaller birth canal, which became a limiting factor on brain size and prompted a shorter gestation period, leaving human offspring unusually immature. Human infants cannot walk much before 12 months, and this dependency reshaped the female reproductive cycle and encouraged alloparenting. The human brain typically reaches 1330 cubic centimeters, nearly three times the size of a chimpanzee or gorilla brain, with the temporal lobes and prefrontal cortex expanding disproportionately. Humans are also unusual in their hands. The ulnar opposition, the contact between thumb and the tip of the little finger, is unique to the genus Homo and underlies the precision grip. The third metacarpal styloid process, which locks the hand bone into the wrist, is not seen in human fossils older than 1.8 million years.

  The oldest known tools are flakes from West Turkana, Kenya, dating to 3.3 million years ago, while the stone tools from Gona, Ethiopia, at about 2.6 million years ago, mark the beginning of the Oldowan technology. A Homo fossil noted at 2.3 million years old was found near some Oldowan tools, hinting that Homo made them, though it is not yet solid evidence. Bernard Wood noted that Paranthropus co-existed with early Homo in the area of the Oldowan Industrial Complex, and in 1994 Randall Susman argued from thumb anatomy that both genera were toolmakers. Susman found that humans have three thumb muscles that chimpanzees lack, along with thicker metacarpals and broader heads for more precise grasping. Until about 50,000 to 40,000 years ago, stone tool use progressed stepwise, with each phase marking a new technology followed by very slow development. Around 50,000 years before present, human culture began to evolve more rapidly, a shift some call the Great Leap Forward or the Upper Palaeolithic Revolution. Modern humans began burying their dead, making clothing from animal hides, hunting with pit traps, and painting caves. Artifacts such as fish hooks, buttons, and bone needles show cultural variation not seen before 50,000 years before present. Recent evidence suggests the Australian Aboriginal population separated from the African population 75,000 years ago and made a 160-kilometer sea journey 60,000 years ago, which may reduce the significance of that revolution.

  The genetic revolution began when Vincent Sarich and Allan Wilson measured immunological cross-reactions of blood serum albumin between humans and African apes. The strength of the reaction gave an immunological distance proportional to the number of amino acid differences between species. In their 1967 paper in Science, Sarich and Wilson estimated the human-ape divergence at four to five million years ago, against standard fossil readings of at least 10 to as much as 30 million years. Later discoveries, notably Lucy, and the reinterpretation of older material such as Ramapithecus, proved the younger estimates correct. A 2012 study in Iceland of 78 children and their parents found a mutation rate of only 36 mutations per generation, pushing the human-chimpanzee split to greater than 7 million years ago. Research on 226 offspring of wild chimpanzees in eight locations suggested chimpanzees reproduce at age 26.5 years on average, placing the divergence between 7 and 13 million years ago. A 2006 comparison of human and chimpanzee genomes found that the X chromosomes diverged about 1.2 million years more recently than the others, suggesting two splits with interbreeding in between. That hybridization startled paleoanthropologists, who nonetheless treated the genetic data seriously, and it left the precise root of the family tree still open to revision.