Human evolution is that part of biological evolution concerning the emergence of humans as a distinct species. It is the subject of a broad scientific inquiry that seeks to understand and describe how this change and development occurred. The term "human," in the context of human evolution, refers to the genus Homo or more specifically the species Homo sapiens, but studies of human evolution usually include other hominins (humans, chimpanzees, and extinct ancestors), such as the australopithecines..
Human evolution theory deals with both the pattern of evolution (descent with modification, the non-causal relations between ancestral and descendant species) and the process of evolution (various theories involving mechanisms and causes for the pattern observed, including fundamental concepts such as natural selection, punctuated equilibrium, and design, and specific scenarios, such as those involving movement from trees, use of tools, "out of Africa," etc).
Substantial evidence has been marshaled for the fact that humans have descended from common ancestors by a process of branching (descent with modification) and for a primate origin of humans. However, proposals for the specific ancestral-descendant relationships and for the process leading to humans tend to be speculative. And, while the theory of natural selection typically is central to scientific explanations for the process, evidence for natural selection being the directive or creative force is limited to extrapolation from the microevolutionary level (changes within the level of species).
Historically, a major source of controversy has been the process by which humans have developed, whether by a force with a random component (natural selection) or by the creative force of a Creator God. Abrahamic religions believe in a single-point origin of modern humans, beginning with an Adam and Eve into whom God breathed life (added a spirit or soul to the material body).
This article will deal mainly with the pattern (descent with modification) of human evolution and not proposed mechanisms or causal explanations. The study of human evolution encompasses many scientific disciplines, most notably physical anthropology, linguistics, and genetics.
- Hominoid: A hominoid is a member of the primate superfamily Hominoidea. The hominoids consist of the various species of gibbons (or "lesser apes"), as well as gorillas, chimpanzees, bonobos, orangutans, and humans (collectively referred to as the "great apes").
- Hominid: The term hominid technically refers to any member of the biological family Hominidae (the "great apes"), a group of primates that includes the extinct and extant humans, chimpanzees, gorillas, and orangutans. Originally, however, the term hominid was restricted to humans and their extinct relatives—those more closely related to humans than the other great apes, which were in a different family—and this definition of hominid is still used by many anthropologists and lay people.
- Hominin: A hominin is a member of the tribe Hominini: extinct and extant chimpanzees and humans.
- Hominan: A hominan is a member of the sub-tribe Hominina: humans and their extinct relatives.
There is substantial evidence for a primate origin of humans (Mayr 2001):
- Anatomical evidence: Human beings exhibit close anatomical similarities with the African apes, and particularly the chimpanzee. Compared to apes, the few unique physical characteristics of humans are the proportion of arms and legs, opposable thumbs, body hair, skin pigmentation, and size of the central nervous system, such as the forebrain.
- Fossil evidence: Numerous fossils have been found sharing human and primate characteristics.
- Molecular evidence: Human molecules are very similar to that of chimpanzees. In some, such as hemoglobin, they are virtually identical.
Indeed, the anatomical and biochemical similarity between chimpanzees and humans is so striking that some scientists have proposed that the two chimpanzee species, troglodytes and paniscus, belong with sapiens in the genus Homo, rather than in Pan. For example, comparisons between chimpanzees and humans in terms of protein sequences, allele differences, and DNA heteroduplex melting points show more than 98 percent identity (King and Wilson 1975; Wood 2006). Ebersberger et al. (2002) found a difference of only 1.24 percent when he aligned 1.9 million nucleotides of chimpanzee DNA and compared them with the corresponding human sequences in the human genome (Wood 2006). Other researchers found similar genetic similarities. Using a 4.97 million nucleotide portion of DNA from human chromosome 7 and comparing to chimpanzee orthologies yielded only 1.13 percent mismatches (Liu et al. 2003). A comparison of a rough draft of the chimpanzee genome with the human genome, for those sequences that could be aligned, averaged 1.23 percent nucleotide mismatches (The Chimpanzee Sequencing and Analysis Consortium 2005). Comparison of chimpanzee exons and human sequences yielded only 0.6 to 0.87 percent differences (Wildman et al. 2003; Nielsen et al. 2005).
Of course, the differences between apes and humans are likewise remarkable. Physiologically, chimpanzees rarely have heart attacks, are resistant to malaria caused by Plasmodium falciparum, and do not go through menopause (Wood 2006). Many cancers common to humans are not found in chimpanzees, and differences exist in terms of the genes for smell and also those that regulate the metabolism of amino acids. Behaviorally, human adult females, unlike any other species, are willing and capable of having sexual intercourse even during those times when they are not able to conceive and become pregnant. Beyond these physical measures, unlike chimpanzees, humans have complex languages (involving syntax and grammar), use symbols in communication, have developed complex technologies, practice and transmit religious belief, conduct scientific experiments, and so forth. Indeed, if one were to overlook the anatomical similarity, the gap between chimpanzees and humans in terms of culture, mental capacity, and various spiritual, emotional, and technological aspects is so large as to dwarf differences between chimpanzees and other animals. Theologically, humans are considered to be distinct from other animals.
Evidence and interpretation
There are two major scientific challenges in deducing the pattern of human evolution.
For one, the fossil record remains fragmentary. Mayr (2001) notes that no fossils of hominids have been found for the period between 6 and 13 million years ago (mya), the time when branching between the chimpanzee and human lineages is expected to have taken place. (While hominid refers to members of the "great ape" family, Hominidae, consisting of extinct and extant gorillas, chimpanzees, orangutans, and humans, Mayr appears to use it in the common anthropological content of animals more closely related to humans than other great apes, such as australopithecines.) Furthermore, as Mayr notes "most hominid fossils are extremely incomplete. They may consist of part of a mandible, or the upper part of a skull without face and teeth, or only part of the extremities." Even the famous "Lucy" finding (Australopithecus afarensis) was only a 40 percent complete female skeleton and lacked a head (Gould 1994).
Coupled with this is a recurrent problem that interpretation of fossil evidence is heavily influenced by personal beliefs and prejudices. Fossil evidence often allows a variety of interpretations, since the individual specimens may be reconstructed in a variety of ways (Wells 2000). As Mayr (2001) notes, "subjectivity is inevitable in the reconstruction of the missing parts," and virtually all hominid finds and interpretations are "somewhat controversial!" Wells (2000) recounts several examples where the pieces of fossils found offered a variety of reconstructions that were sometimes dramatically different, such as long face versus a short face, a heavy brow, a missing forehead. Different interpretations of two sections of a fossil skull and how to place one of those pieces led Roger Lewin to recount, "How you held it really depended on your preconceptions. It was very interesting what people did with it" (Wells 2000).
History of paleoanthropology
The modern field of paleoanthropology (study of human origins) began in the nineteenth century with the discovery of "Neanderthal man" (the eponymous skeleton was found in 1856, but there had been finds elsewhere since 1830). However, fossils of Neanderthal's were widely misinterpreted as skeletons of modern humans with deformation or disease (Gould 1990).
The idea that humans are similar to certain great apes had been obvious to people for some time, but the idea of the biological evolution of species in general was not significantly advanced until after Charles Darwin published On the Origin of Species in 1859. Though Darwin's first book on evolution did not address the specific question of human evolution— "light will be thrown on the origin of man and his history" was all Darwin wrote on the subject— the implications of evolutionary theory were clear to contemporary readers. Debates between Thomas Huxley and Richard Owen focused on the idea of human evolution. Huxley convincingly illustrated many of the similarities and differences between humans and apes in his 1863 book Evidence as to Man's Place in Nature. By the time Darwin published his own book on the subject, Descent of Man, it was already a well-known interpretation of his theory—and the interpretation helped make the theory of natural selection highly controversial. Even many of Darwin's original supporters (such as Alfred Russel Wallace and Charles Lyell) balked at the idea that human beings could have evolved their apparently boundless mental capacities and moral sensibilities through natural selection.
Since the time of Carolus Linnaeus, the great apes were considered the closest relatives of human beings, based on morphological similarity. In the nineteenth century, it was speculated that their closest living relatives were chimpanzees and gorillas, and based on the natural range of these creatures, it was surmised humans share a common ancestor with other African apes and that fossils of these ancestors would ultimately be found in Africa.
It was not until the 1920s that hominin fossils were discovered in Africa. In 1924, Raymond Dart described Australopithecus africanus. The type specimen was the Taung Child, an australopithecine infant discovered in a cave deposit being mined for concrete at Taung, South Africa. The remains were a remarkably well-preserved tiny skull and an endocranial cast of the individual's brain. Although the brain was small (410 cm3), its shape was rounded, unlike that of chimpanzees and gorillas, and more like a modern human brain. Also, the specimen exhibited short canine teeth, and the position of the foramen magnum (the hole in the skull where the spine enters) was evidence of bipedal locomotion. All of these traits convinced Dart that the Taung baby was a bipedal human ancestor, a transitional form between apes and humans. Another 20 years would pass before Dart's claims were taken seriously, following the discovery of more fossils that resembled his find. The prevailing view of the time was that a large brain evolved before bipedality. It was thought that intelligence on par with modern humans was a prerequisite to bipedalism.
The australopithecines, which now are widely seen as direct ancestors leading to the human lineage, have been classified as either gracile (Australopithecus genus) or robust (Paranthropus genus). However, some consider the robust species aethiopicus, boisei, and robustus as included within the genus Australopithecus. In the 1930s, when the robust specimens were first described, the Paranthropus genus was used. During the 1960s, the robust variety was moved into Australopithecus. The recent trend has been back to the original classification as a separate genus.
The evolutionary history of the primates can be traced back for some 60 million years, as one of the oldest of all surviving placental mammal groups. Many paleontologists consider that primates share a common ancestor with the bats, another extremely ancient lineage, and that this ancestor probably lived during the late Cretaceous together with the last dinosaurs. The oldest known primates come from North America, but they were widespread in Eurasia and Africa as well, during the tropical conditions of the Paleocene and Eocene.
With the beginning of modern climates, marked by the formation of the first Antarctic ice in the early Oligocene around 40 million years ago, primates went extinct everywhere but Africa and southern Asia.
|Paleocene epoch||Eocene epoch||Oligocene epoch|
|Danian | Selandian
|Ypresian | Lutetian
Bartonian | Priabonian
|Rupelian | Chattian|
Fossil evidence found in Germany 20 years ago (Kordon and Begun 2001, Heizmann and Begun 2001) was determined to be about 16.5 million years old, some 1.5 million years older than similar species from East Africa. It suggests that the great ape and human lineage first appeared in Eurasia and not Africa. The discoveries suggest that the early ancestors of the hominids (the family of great apes and humans) migrated to Eurasia from Africa about 17 million years ago, just before these two continents were cut off from each other by an expansion of the Mediterranean Sea.
Heizmann and Begun (2001) hold that the great apes flourished in Eurasia and that their lineage leading to the African apes and humans—Dryopithecus—migrated south from Europe or Western Asia into Africa. The surviving tropical population, which is seen most completely in the upper Eocene and lowermost Oligocene fossil beds of the Fayum depression southwest of Cairo, is held to have given rise to all living primates—lemurs of Madagascar, lorises of Southeast Asia, galagos or "bush babies" of Africa, and the anthropoids: platyrrhines or New World monkeys, and catarrhines or Old World monkeys and the great apes and humans.
The earliest known catarrhine, or Old World monkey, is Kamoyapithecus from uppermost Oligocene at Eragaleit in the northern Kenya rift valley, dated to 24 mya (millions years ago). Its ancestry is generally thought to be close to such genera as Aegyptopithecus, Propliopithecus, and Parapithecus from the Fayum, at around 35 mya. There are no fossils from the intervening 11 million years.
No near ancestor to South American platyrrhines, whose fossil record begins at around 30 Ma, can be identified among the North African fossil species, and possibly lies in other forms that lived in West Africa that were caught up in the still-mysterious transatlantic sweepstakes that sent primates, rodents, boa constrictors, and cichlid fishes from Africa to South America sometime in the Oligocene.
In the early Miocene, after 22 mya, many kinds of arboreally adapted primitive catarrhines from East Africa appeared. Because the fossils at 20 mya include fragments attributed to Victoriapithecus, the earliest cercopithecoid, the other forms are (by default) grouped as hominoids, without clear evidence as to which are closest to living apes and humans. Among the presently recognized genera in this group, which ranges up to 13 mya, one finds Proconsul, Rangwapithecus, Dendropithecus, Limnopithecus, Nacholapithecus, Equatorius, Nyanzapithecus, Afropithecus, Heliopithecus, and Kenyapithecus, all from East Africa. The presence of other generalized non-cercopithecids of middle Miocene age from sites far distant—Otavipithecus from cave deposits in Namibia, and Pieroloapithecus and Dryopithecus from France, Spain, and Austria—is evidence of a wide diversity of forms across Africa and the Mediterranean basin during the relatively warm and equable climatic regimes of the early and middle Miocene. The youngest of the Miocene hominoids, Oreopithecus, is from 9 mya coal beds in Italy.
|Tertiary sub-era||Quaternary sub-era|
Gibbons and orangutans. Molecular evidence indicates that the lineage of gibbons (family Hylobatidae) became distinct between 18 and 12 mya, and that of orangutans (subfamily Ponginae) at about 12 mya. We have no fossils that clearly document the ancestry of gibbons, which may have originated in a so far unknown South East Asian hominoid population, but fossil proto-orangutans may be represented by Ramapithecus from India and Griphopithecus from Turkey, dated to around 10 mya.
Gorillas, chimpanzees, and hominans. Molecular evidence further suggests that between 8 and 4 mya, first the gorillas, and then the chimpanzee (genus Pan) split off from the line leading to the modern humans. However, there is no fossil record of either group of African great apes. One explanation is that bones do not fossilize well in rainforest environments. Patterson et al. (2006) reported on genetic evidence that humans and chimpanzees probably diverged less than 6.3 mya. More controversially, they speculated on the basis of the evidence that after divergence of the lineages, interbreeding again took place, before the final split about 5.3 mya.
Hominines, however, seem to have been one of the mammal groups (as well as antelopes, hyenas, dogs, pigs, elephants, and horses) that adapted to the open grasslands as soon as this biome appeared, due to increasingly seasonal climates, about 8 mya, and their fossils are relatively well known. The earliest are Sahelanthropus tchadensis (7-6 mya) and Orrorin tugenensis (6 mya), followed by:
- Ardipithecus (5.5-4.4 mya), with species Ar. kadabba and Ar. ramidus;
- Australopithecus (4-2 mya), with species Au. anamensis, Au. afarensis, Au. africanus, Au. bahrelghazali, and Au. garhi;
- Paranthropus (3-1.2 mya), with species P. aethiopicus, P. boisei, and P. robustus;
- Homo (2 mya-present).
Australopithecus afarensis and Australopithecus africanus are among the most famous of the extinct hominids. The australopithecines are now thought to be immediate ancestors of the genus Homo, the group to which modern humans belong. Both australopithecines and Homo sapiens are part of the tribe Hominini.
As noted above, the australopithecines have been classified as either gracile or robust. Australopithecus is the genus into which the gracile (slender) australopithecines are placed. They appeared earlier in the fossil record than Paranthropus (robust australopithecines), which is believed to have developed from the ancestral Australopithecus line. The three recognized species of the Paranthropus genus are aethiopicus, boisei, and robustus, but the species aethiopicus may not be distinguishable from boisei (Mayr 2001). The robust species aethiopicus, boisei, and robustus are at times included within the genus Australopithecus, rather than recognize the separate genus of Paranthropus.
The genus Homo
In modern taxonomy, Homo sapiens is the only extant (living) species of its genus, Homo. However, the ongoing study of the origins of Homo sapiens often demonstrates that there were other Homo species, all of which are now extinct. While some of these other species might have been ancestors of H. sapiens, many were likely "cousins," having speciated away from our ancestral line. There is not yet a consensus as to which of these groups should count as separate species and which as subspecies of another species. In some cases, this is due to the paucity of fossils; in other cases, it is due to the slight differences used to classify species in the Homo genus.
The word homo is Latin for "person," chosen originally by Carolus Linnaeus in his classification system. It is often translated as "man," although this can lead to confusion, given that the English word "man" can be generic like homo, but can also specifically refer to males. Latin for "man" in the gender-specific sense is vir, cognate with "virile" and "werewolf." The word "human" is from humanus, the adjectival form of homo.
H. habilis lived from about 2.4 to 1.5 million years ago (mya). H. habilis, the oldest known species of the genus Homo, appeared in South and East Africa in the late Pliocene or early Pleistocene, 2.5–2 mya, when it is considered to have diverged from the Australopithecines. H. habilis had smaller molars and larger brains than the Australopithecines, and made tools from stone and perhaps animal bones. One of the first known hominids, it was nicknamed "handy man" by its discoverer, Louis Leakey. Some scientists have proposed moving this species out of Homo and into Australopithecus.
Homo rudolfensis and Homo georgicus
These are proposed species names for fossils from about 1.9-1.6 mya, the relation of which with H. habilis is not yet clear.
- H. rudolfensis refers to a single, incomplete skull from Kenya.
- H.georgicus, from the country of Georgia, may be an intermediate form between H. habilis and H. erectus.
Homo ergaster and Homo erectus
The first fossils of Homo erectus were discovered by Dutch physician Eugene Dubois in 1891 on the Indonesian island of Java. He originally gave the material the name Pithecanthropus erectus based on its morphology, which he considered to be intermediate between that of humans and apes.
H. erectus lived from about 1.8 mya to 70,000 years ago. Often the early phase, from 1.8 to 1.25 mya, is considered either to be a separate species, H. ergaster, or it is seen as a subspecies of erectus, Homo erectus ergaster.
Homo erectus was apparently very successful, with fossils found in Africa, Asia (Indonesia and China), Georgia (Caucasus region of Europe), and eastern and southern Africa (Mayr 2001). It is considered to be the first hominid to spread out of Africa. The differences between the early populations of H. erectus, found in Africa, and the later populations, found in Asia, Europe, and Africa, are substantial enough for the separation by many researchers into the early African H. ergaster and the mainly Asian populations H. erectus (Smithsonian 2007b).
In the Early Pleistocene, 1.5–1 mya, in Africa, Asia, and Europe, presumably, Homo habilis evolved larger brains and made more elaborate stone tools; these differences and others are sufficient for anthropologists to classify them as a new species, H. erectus. In addition H. erectus was the first human ancestor to walk truly upright. This was made possible by the development of locking knees and a different location of the foramen magnum (the hole in the skull where the spine enters). They may have used fire to cook their meat.
A famous example of Homo erectus is Peking Man.
Homo cepranensis and Homo antecessor
These are proposed as species that may be intermediate between H. erectus and H. heidelbergensis.
- H. cepranensis refers to a single skull cap from Italy, estimated to be about 800,000 years old.
- H. antecessor is known from fossils from Spain and England that are 800,000-500,000 years old.
H. heidelbergensis (Heidelberg Man) lived from about 800,000 to about 300,000 years ago. Also proposed as Homo sapiens heidelbergensis or Homo sapiens paleohungaricus.
H. neanderthalensis lived from about 250,000 to as recent as 30,000 years ago. Also proposed as Homo sapiens neanderthalensis: There is ongoing debate over whether the "Neanderthal Man" was a separate species, Homo neanderthalensis, or a subspecies of H. sapiens. While the debate remains unsettled, the prevailing view of evidence, collected by examining mitochondrial DNA and Y-chromosomal DNA, currently indicates that little or no gene flow occurred between H. neanderthalensis and H. sapiens, and therefore, the two were separate species.
Krings et. al. (1997) found in genetic studies that Neanderthals and modern humans diverged genetically 500,000 to 600,000 years ago, suggesting that though they may have lived at the same time, Neanderthals did not contribute genetic material to modern humans. One of the participants of this study concluded: "These results [based on mitochondrial DNA extracted from Neanderthal bone] indicate that Neanderthals did not contribute mitochondrial DNA to modern humans… Neanderthals are not our ancestors" (PSU 1997). Subsequent investigation of a second source of Neanderthal DNA supported these findings. However, supporters of the multiregional hypothesis point to recent studies indicating non-African nuclear DNA heritage dating to one mya, as well as apparent hybrid fossils found in Portugal and elsewhere, in rebuttal to the prevailing view.
Homo rhodesiensis, and the Gawis cranium
- H. rhodesiensis, estimated to be 300,000-125,000 years old, most current experts believe Rhodesian Man to be within the group of Homo heidelbergensis though other designations such as Archaic Homo sapiens and Homo sapiens rhodesiensis have also been proposed.
- In February 2006, a fossil, the Gawis cranium, was found that might possibly be a species intermediate between H. erectus and H. sapiens or one of many evolutionary dead ends. The skull from Gawis, Ethiopia, is believed to be 500,000-250,000 years old. Only summary details are known, and no peer reviewed studies have been released by the finding team. Gawis man's facial features suggest it's either an intermediate species or an example of a "Bodo man" female (IU 2006).
H. sapiens ("sapiens" means wise or intelligent) have lived from about 250,000 years ago to the present. Between 400,000 years ago and the second interglacial period in the Middle Pleistocene, around 250,000 years ago, the trend in cranial expansion and the elaboration of stone tool technologies developed, providing evidence for a transition from H. erectus to H. sapiens.
The direct evidence suggests there was a migration of H. erectus out of Africa, then a further speciation of H. sapiens from H. erectus in Africa (there is little evidence that this speciation occurred elsewhere). Then a subsequent migration within and "out of Africa" eventually replaced the earlier dispersed H. erectus. This migration and origin theory is usually referred to as the single-origin theory ("Out of Africa Hypothesis"). However, the current evidence does not preclude multiregional speciation, either. This is a hotly debated topic in paleoanthropology.
Based on molecular evidence, the calculation of the time of divergence of all modern human populations from a common ancestor typically yields dates around 200,000 years (Smithsonian 2007a). The oldest fossil evidence for modern humans is 130,000 years old in Africa and sometime before 90,000 years old in the Near East (Smithsonian 2007a). Some consider H. sapiens idaltu from Ethiopia, which lived from about 160,000 years ago, to be the oldest known anatomically modern human.
Notably, however, about 50,000 to 40,000 years ago, human beings appeared to have taken a Great Leap Forward, when human culture apparently changed at a much greater speed. It seems connected to the arrival of modern humans beings: Homo sapiens sapiens. (See modern man and the great leap forward.)
The Cro-Magnons form the earliest known European examples of Homo sapiens sapiens. The term falls outside the usual naming conventions for early humans and is used in a general sense to describe the oldest modern people in Europe. Cro-Magnons lived from about 40,000 to 10,000 years ago in the Upper Paleolithic period of the Pleistocene epoch. For all intents and purposes these people were anatomically modern, only differing from their modern day descendants in Europe by their slightly more robust physiology and larger brain capacity than that of modern humans. When they arrived in Europe about 40,000 years ago, they brought with them sculpture, engraving, painting, body ornamentation, music, and the painstaking decoration of utilitarian objects.
Current research establishes that human beings are highly genetically homogeneous, meaning that the DNA of individual Homo sapiens is more alike than usual for most species. Distinctive genetic characteristics have arisen, however, primarily as the result of small groups of people moving into new environmental circumstances. Such small groups are initially highly inbred, allowing the relatively rapid transmission of traits favorable to the new environment. These adapted traits are a very small component of the Homo sapiens genome and include such outward "racial" characteristics as skin color and nose form in addition to internal characteristics such as the ability to breathe more efficiently in high altitudes.
Abrahamic religions believe in a single-point origin of modern humans, beginning with an original couple, Adam and Eve, into whom God breathed life (added a spirit or soul to the material body).
Mitochondrial Eve, Y-chromosomal Adam, and the most recent common ancestor
Geneticists have identified individuals dubbed "Mitochondrial Eve" and "Y-chromosomal Adam," with Mitochondrial Eve the common matrilineal ancestor of all humans alive today and Y-chromosomal Adam the common patrilineal ancestor who lived many millennia after Mitochondrial Eve. (That a common patrilineal ancestor should have lived significantly after a matrilineal one is in concordance with Bible history, which gives Noah as a more recent common patrilineal ancestor than Adam.) In the founding work of this area of investigation, Cann et al. (1987) compared mitochondrial DNA sampled from women whose ancestors came from different part of the world. and concluded that Africa was the most likely root of human ancestry and that human ancestors left Africa between between 180,000 and 230,000 years ago.
Note that Mitochondrial Eve is the most recent common matrilineal ancestor, and Y-chromosomal Adam the most common patrilineal ancestor, not the original man and woman, and not the most recent common ancestor (MRCA) of all humans. All living humans can trace their ancestry back to the MRCA via at least one of their parents. That is, the MRCA's offspring have led to all living humans via sons and daughters. However, Mitochondrial Eve must be traced only through female lineages, so she is estimated to have lived much longer ago than the MRCA. While Mitochondrial Eve is thought to have been living around 140,000 years ago (Dawkins 2004) or between 100,000 and 200,000 years ago (Rhode 2005), and Y-chromosomal Adam about 35,000 to 89,000 years ago (Rhode 2005), the MRCA is estimated to have been living only around 2,000 to 5,000 years ago (Rohde et al. 2004, Rhode 2005).
Starting with "the" MRCA at around 3,000 years ago, one can trace all ancestors of the MRCA backward in time. At every ancestral generation, more and more ancestors (via both paternal and maternal lines) of MRCA are found. These ancestors are by definition also common ancestors of all living people. Eventually, there will be a point in the past where all humans can be divided into two groups: Those who left no descendants today and those who are common ancestors of all living humans today. This point in time is termed the "identical ancestors point" and is estimated to be between 5,000 and 15,000 years ago. Since Mitochondrial Eve is estimated to have lived more than hundred thousand years before the identical ancestors point, every contemporary woman of hers is either not an ancestor of all living people, or a common ancestor of all living people (Dawkins 2004; Rohde 2005).
H. floresiensis is the name given to a proposed species that lived about 100,000-12,000 years ago (announced October 28, 2004 in the science journal Nature). It is based on fossil findings, with the main find a fossil believed to be a woman about 30 years of age. The fossil was dated to approximately 18,000 years ago. Her brain size was only 380 cm3 (which can be considered small even for a chimpanzee), and she was only 1 meter in height. The species has been nicknamed "hobbit" for its small size, probably a result of insular (island) dwarfism.
H. floresiensis is intriguing both for its size and its age, being a concrete example of what some consider a recent species of the genus Homo that exhibits derived traits not shared with modern humans. In other words, H. floresiensis is postulated to share a common ancestor with modern humans, but split from the modern human lineage and followed a distinct evolutionary path.
However, there is an ongoing debate over whether H. floresiensis is indeed a separate species. Some scientists presently believe that H. floresiensis was a modern H. sapiens suffering from pathological dwarfism. This hypothesis is supported in part, because the modern humans who live on Flores, the island where the fossil was found, are pygmies. This coupled with pathological dwarfism could indeed create a hobbit-like human. The other major attack on H. floresiensis is that it was found with tools only associated with H. sapiens.
Comparative table of Homo species
- Bolded species names indicate the existence of numerous fossil records.
|species||lived when (MYA)||lived where||adult length (m)||adult weight (kg)||brain volume (cm³)||fossil record||discovery / publication of name|
|H. rudolfensis||1.9||Kenya||1 skull||1972/1986|
|H. ergaster||1.9–1.25||E. and S. Africa||1.9||700–850||many||1975|
|H. erectus||2(1.25)–0.3||Africa, Eurasia (Java, China, Caucasus)||1.8||60||900–1100||many||1891/1892|
|H. cepranensis||0.8?||Italy||1 skull cap||1994/2003|
|H. antecessor||0.8–0.35||Spain, England||1.75||90||1000||3 sites||1997|
|H. heidelbergensis||0.6–0.25||Europe, Africa, China||1.8||60||1100–1400||many||1908|
|H. neanderthalensis||0.23–0.03||Europe, W. Asia||1.6||55–70 (heavily built)||1200-1700||many||(1829)/1864|
|H. rhodesiensis||0.3–0.12||Zambia||1300||very few||1921|
|H. sapiens sapiens||0.25–present||worldwide||1.4–1.9||55–80||1000–1850||still living||—/1758|
|H. sapiens idaltu||0.16||Ethiopia||1450||3 craniums||1997/2003|
|H. floresiensis?||0.10–0.012||Indonesia||1.0||25||400||7 individuals||2003/2004|
Out of Africa and multiregional positions
There are two dominant, and one might say polarizing, general views on the issue of human origins, the Out of Africa position and the multiregional position.
The Out of Africa, or Out of Africa II, or replacement model holds that after there was a migration of Homo erectus (or H. ergaster) out of Africa and into Europe and Asia, these populations did not subsequently contribute significant amounts of genetic material (or, some say, contributed absolutely nothing) to later populations along the lineage to Homo sapiens (Kreger 2005). Later, approximately 200,000 years ago, there was a second migration of hominids out of Africa, and this was modern H. sapiens that replaced the populations that then occupied Europe and Asia (Kreger 2005). This view maintains a specific speciation event that led to H. sapiens in Africa, and this is the modern human.
The multiregional or continuity camp hold that since the origin of H. erectus, there have been populations of hominids living in the Old World and that these all contributed to successive generations in their regions (Kreger 2005). According to this view, hominids in China and Indonesia are the most direct ancestors of modern East Asians, those in Africa are the most direct ancestors of modern Africans, and the European populations either gave rise to modern Europeans or contributed significant genetic material to them, while their origins were in Africa or West Asia (Kreger 2005). There is genetic flow to allow for the maintenance of one species, but not enough to prevent racial differentiation.
There are various combinations of these ideas. Overall, the disagreement between these two camps has caused "fairly severe strife within the paleoanthropological community" (Kreger 2005). Multiregionalism is often "portrayed as a racist theory," while Out of Africa II "has often been portrayed as a religiously motivated idea" that strives to align with the biblical story of Genesis (Kreger 2005).
Use of tools
Using tools is a sign of intelligence. Some scientists speculate that tool use also played a role in the origin of modern humans. Over the past 2 or 3 million years, human brain size has increased threefold. A brain needs a lot of energy: The brain of a modern human uses about 20 Watts (about 400 calories per day), one fifth of total human energy consumption. Early hominoids, like apes, were considered to have been essentially plant eaters (fruit, leaves, roots), their diet only occasionally supplemented by meat (often from scavenging). However, plant food in general yields considerably less energy and nutritive value than meat. Therefore, being able to hunt for large animals, which was only possible by using tools such as spears, would have made it possible for humans to sustain larger and more complex brains, which in turn allowed them to develop yet more intelligent and efficient tools.
Precisely when early humans started to use tools is difficult to determine, because the more primitive these tools were (for example, sharp-edged stones), the more difficult it is to decide whether they are natural objects or human artifacts. There is some evidence that the australopithecines (4 mya) may have used broken bones as tools, but this is debated.
Stone tools are first attested around 2.6 mya, when H. habilis in Eastern Africa used so-called pebble tools: Choppers made out of round pebbles that had been split by simple strikes. This marks the beginning of the Paleolithic, or Old Stone Age; its end is argued to be the end of the last Ice Age, around 10,000 years ago. The Paleolithic is subdivided into the Lower Paleolithic (Early Stone Age, ending around 350,000–300,000 years ago), the Middle Paleolithic (Middle Stone Age, until 50,000–30,000 years ago), and the Upper Paleolithic.
The period from 700,000–300,000 years ago is also known as the Acheulean, when H. ergaster (or erectus) made large stone hand-axes out of flint and quartzite, at first quite rough (Early Acheulian), later "retouched" by additional, more subtle strikes at the sides of the flakes.
After 350,000 BP (Before Present), a more refined, the so-called Levallois, technique was developed. It consisted of a series of consecutive strikes, by which scrapers, slicers ("racloirs"), needles, and flattened needles were made. Finally, after about 50,000 BP, ever more refined and specialized flint tools were made by the Neanderthals and the immigrant Cro-Magnons (knives, blades, skimmers). In this period, they also started to make tools out of bone.
The "modern man" debate and the Great Leap Forward
Until about 50,000–40,000 years ago, the use of stone tools seems to have progressed stepwise: Each phase (habilis, ergaster, and neanderthal) started at a higher level than the previous one, but once that phase had started, further development was slow. In other words, one might call these Homo species culturally conservative.
After 50,000 BP, in what Jared Diamond, author of The Third Chimpanzee, and other anthropologists characterize as a "Great Leap Forward," human culture apparently started to change at much greater speed: "Modern" humans started to bury their dead carefully, made clothing out of hides, developed sophisticated hunting techniques (such as pitfall traps, or driving animals to fall off cliffs), and made cave paintings. This speed-up of cultural change seems connected with the arrival of modern humans, Homo sapiens sapiens.
Additionally, human culture began to become more technologically advanced, in that different populations of humans begin to create novelty in existing technologies. Artifacts such as fish hooks, buttons, and bone needles begin to show signs of variation among different population of humans, something what had not been seen in human cultures prior to 50,000 BP.
Theoretically, modern human behavior is taken to include four ingredient capabilities: Abstract thinking (concepts free from specific examples), planning (taking steps to achieve a farther goal), innovation (finding new solutions), and symbolic behavior (such as images, or rituals). Among concrete examples of modern human behavior, anthropologists include specialization of tools, use of jewelry and images (such as cave drawings), organization of living space, rituals (for example, burials with grave gifts), specialized hunting techniques, exploration of less hospitable geographical areas, and barter trade networks. Debate continues whether there was indeed a "Revolution" leading to modern humanity ("the big bang of human consciousness"), or a more gradual evolution.
This list will conduct in chronological order, following genus.
- Sahelanthropus tchadensis
- Orrorin tugenensis
- Ardipithecus kadabba
- Ardipithecus ramidus
- Australopithecus anamensis
- Australopithecus afarensis
- Australopithecus bahrelghazali
- Australopithecus africanus
- Australopithecus garhi
- Paranthropus aethiopicus
- Paranthropus boisei
- Paranthropus robustus
- Kenyanthropus platyops
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