Michael Roberts
Department of Biology
Linfield College
Outline:
a. Models of the origin and dispersal of Homo sapiens
b. When, where, and how did the step to moderns occur?
c. Replacement vs. Regional continuity hypotheses: implications for human
nature
Early Hominids: The first fossils of erect bipeds with our general body shape (called hominids) appear in Central Africa about 6.5 million years ago (mya). Until recently, the hominids were thought to have been limited to East Africa, but a recent find from Chad, called Sahelanthropus, indicates that the early group of hominids is quite diverse, both anatomically and geographically. Sahelanthropus is so far represented only by skulls and teeth, but is clearly a hominid because of the small teeth, (especially the canines), the short face, and upright posture (estimated from the position of the foramen magnum). The next hominids come from East Africa and are dated to between 5 and 2 mya. They were represented by Ardipithecus and Kenyanthropus, followed by Australopithecus. This last genus contains the most famous of the early hominids -- the "Lucy" skeleton discovered by Johansen in 1974 (Figure 1) and the remarkable footprints discovered by Leakey in 1978. These hominids are intermediate between modern humans and apes, with a small brain and slightly elongated arms, though with completely upright posture and walking gait. They are not known to have used fire or made tools. They are also not known to have ventured out of Eastern and Southern Africa.
Even though they have been divided by some investigators into several genera (Sahelanthropus, Ardipithecus, Australopithecus, and Paranthropus),it is still unclear whether the early hominids were simply diverse within a widely-distributed group (for example, like modern humans), or whether they represent more than one evolutionary line. For example, the discovery of Kenyanthropus has prompted the suggestion by Leakey et al. that this genus, rather than Australopithecusas known by the Lucy material, was the precursor of later hominids. They propose that Kenyanthropuswas ancestral to Homo rudolfensis,already known from East Africa. Further study will be required to confirm or refute this idea. A comparison of Kenyanthropusand H. rudolfensis is included here.
By 2 mya, the first members of the genus Homo, called Homo habilis, had evolved in Africa (Figure 2). They did make tools, though it is not known whether they used fire. Homo erectus seems to have rapidly spread throughout Africa, Europe, and Asia (Figure 3), (Figure 4) . Recent discoveries of hominid fossils in Georgia (between the Black Sea and the Caspian Sea), dated to 1.75 mya, have made the question of the migration out of Africa more complex, since these fossils resemble Homo habilis as much as they resemble Homo erectus, and they have left tools that look like those of Homo habilis in Africa.
The European history of Homo begins about 800 thousand years ago (kya), when Homo erectus moved into Spain (Figure 5) and inhabited at least the north-eastern part of the country. These people are also found in southern France at a later date (450 kya). Other skulls, jaws, and long bones have been found in Germany and the UK, and may represent continued evolution of Homo erectus in Europe. One remarkable find is the Mauer Jaw (Figure 6) , a large mandible with all its teeth. It is often classed in a separate species, Homo heidelbergensis,but is most likely a large version of Homo erectus. They were fine tool-makers, making handaxes called "Acheulian", after the French locale -- St. Acheul -- where they were first found; they also left much evidence that they used fire.
Neanderthals, the descendants of Homo erectus in Europe and Western Asia: The Homo erectus populations in Europe and the Near East lived for a half million years in Europe, then disappeared or evolved into the so-called "Archaics". Though these archaics are poorly represented in the fossil record, they may be the ancestors of the Neanderthals.
The Neanderthals are known from hundreds of specimens collected from hundreds of localities from Western Europe to Western Asia. They are not known from Africa or Eastern Asia.
Characteristics of Neanderthals:
The oldest modern humans are found in Africa, and are dated about 130 kya. They clearly evolved from local African populations of Homo erectus.All paleoanthropologists agree that such local evolution had to occur in at least one place, but the argument about the origins of modern groups of humans centers on what happened next. The next step in human evolution either involved
Œ‡ the initial evolutionary step of Homo erectusin Africa to Homo sapiensin Africa was followed by a migration of Homo sapiensover the rest of the world, with the previous local populations of Homo erectusin Asia going extinct (the Replacement Model).
The Fossil Evidence
We will investigate the evidence for both sides in this very interesting
conflict, but first we will look at the earliest remains of Homo sapiensin Africa and the timing of their appearance in other
locations throughout the Old World.
Omo
Thick skull, but modern in shape; chin
present, limb bones similar to today's east Africans. Could be as old as 130
kya.
Kabwe (Rhodesian Man)
(Figure 7)
Similar to Omo, also modern in shape; cranial capacity about 1300 cc. Date
uncertain, but is at least 125 kya.
Border Cave and Klasies River Mouth
Bones are fragmentary, but show modern
features though fairly large size; fire present. Dates are uncertain, but range
from 80 to 130 kya.
Jebel Irhoud
Robust bones with both modern and archaic
features. Date of 100 to 200 kya.
Near East
In Israel, modern human (Homo sapiens)and Neanderthal bones are found in separate caves
only short distances apart. The dates for the modern human remains (Skhul and
Qafzeh caves) are both earlier and later than those of the Neanderthals (Tabun
and Kebara caves). One of the skulls from the Skhul cave (Skhul V) shows
neanderthaloid features in its prognathism and brow ridges, though other skulls
from the same site do not. It is possible to interpret Skhul V as evidence for interbreeding
between moderns and Neanderthals, though there is no other evidence in the
caves for this possibility. It is not known whether the two groups were present
in the Near East at the same time, but it is clear that anatomically modern
humans predated Neanderthals in the region.
The Neanderthals may have come from the north (from Europe), and the moderns
may have come from the south (from Africa). The Neanderthals had already been
living in Europe for 100 ky, and the modern humans had recently begun to spread
from their African point of origin. These sites in Israel represent the oldest
evidence for potential contact between these two groups.
Asia and
Australia
Chinese bones dating to 200 kya show "modern features", suggesting to Chinese anthropologists that there was a local evolution of Homo erectusto Homo sapiens.Chinese researchers thus propose that there is an unbroken line of evolution between the particular Homo erectusin China and the modern Chinese, as represented by the skull from Dali (Figure 8). However, in the Dali skull and in others of the same age in China, there are thick browridges and the skull vaults are low. Thus it is most likely that the 200 kya Chinese fossils represent a continued evolution of Homo erectusto another archaic form rather than the appearance of modern Homo sapiens. By 25 kya there are undoubted modern Homo sapiensin China, most likely the result of immigration in the previous millenia.
People probably reached Australia about 55 kya, though skeletal material is not found until 40 kya. The very recent bones from Australia (9 to 14 kya) show brow ridges, receding foreheads, and thick bones, features that would link them with Homo erectusbut for their recent age and large cranial capacities.
The New World
Old World people entered America at about 12 kya, though some sites have been interpreted to indicate entry as early as 30 kya. In the absence of better dates, human remains, and undisputed artifacts, these early dates are uncertain. it is not disputed, however, that these people were fully modern.
Europe
The greatest concentration of human fossils in Europe has come from deposits whose dates span the last 130 thousand years. During that time the climate of Europe went through several cool-warm cycles: 130 to 110 kya it was like that of today; 110 to 75 kya it was slightly cooler; 75 to 30 kya it was cool to borderline glacial; and 30 to 12 kya was a full glacial climate. The glacial climate ended abruptly 12,000 years ago. The last 12,000 years have seen smaller fluctuations in climate, most lasting a few hundred to a few thousand years, but none returning to the glacial climae of 20,000 years ago.
During most of the glacial period, the Neanderthals were the only human inhabitants of Europe. In fact, they were the only Europeans from 230 kya until 40 kya. Fully modern people arrived about 40 kya and within 5 ky after that the Neanderthals were gone.
Some sites (e.g Mladec in Czechoslovakia) have produced skeletons with an apparent mixture of Neanderthal and modern characteristics. In others, the moderns appear alone and with completely modern appearance about 40 kya. The best known of these sites are those from the Dordogne region of southern France. The cave of Cro Magnon gave its name to an entire "race" of moderns though the skulls found there were not typical of their contemporaries, nor of today's people. The Cro Magnon cave people were taller than today's humans, with larger cranial capacities (1500 cc, as in Neanderthals).
Tool kit
The Neanderthal culture is generally called the Mousterian, after the cave
location (Le Moustier) where the first tools were found in association with
Neanderthal remains. It is regarded as Middle Paleolithic because the later
tools were of a more advanced (Upper Paleolithic) style. The succeeding
cultures are associated with Homo sapiens.and
began 40 kya with the entry of these moderns into Europe. The earliest Upper
Paleolithic (UP) culture (the Chatelperronian) has been seen as an industry
crafted by Neanderthals under influence from Homo sapiens.The Aurignacian and later cultures are purely Homo
sapiens,for by those times, there were no
Neanderthals: from 30 kya to the present, all cultures and remains are those of
fully modern humans.
The ability to make useful and multiple tools developed slowly until the last 30 kya. The Acheulian handaxe served Homo erectusinitially as the tool itself, then later as the core from which other tools were cut. Advances in tool-making by later people consisted not only of increasing the types of tools, but also of increasing the amount of useful surface that a core could produce.
The modern humans of the UP produced functional and beautifully-flaked blades, punches, burins, needles, wooden and bone tools (the first known) and leather goods. Many of these had designs carved on them (also the first known), indicating a level of artistic expression that is not seen with the Neanderthals (though a suspected bone flute has been found in a Neanderthal site (Figure 9).
Designs were also made on the walls of caves and as sculptures in stone. These works seem to be examples of sympathetic magic useful in the hunt rather than art in the sense that we use it: for example, the cave art is not in easily-accessible parts of the caves, but in parts that are very difficult to find and get to yet had good acoustical properties; most types of animals were not painted -- only those that were either hunted, or dangerous; only men are shown near the animals; the era of cave paintings coincides with the coldest part of the last glacial period.
Recall that there are two primary theories of modern human evolution:
...1. local evolutionary steps in several places, populations of Homo erectus evolving separately to populations of Homo sapiens(the Regional Continuity Model) (Figure 10) or
...2. the initial evolutionary step ofHomo erectusin Africa to Homo sapiensin Africa was followed by a migration of Homo sapiensover the rest of the world, with the previous local populations of Homo erectusin Asia going extinct (the Replacement Model) (Figure 11).
As earlier discussed, paleoanthropologists agree that sometime about 1.5 mya a group of Homo erectusmoved out of Africa into Asia, and eventually into Europe. In those locations, they evolved into local races while still retaining their Homo erectusnature. The various camps part company at this point.
Some anthropologists, best represented by Milford Wolpoff and Alan Templeton, propose that the local groups of Homo erectusin each part of the world evolved separately into Homo sapiens.Separate evolutionary steps generally should create separate species, but Wolpoff explains the emergence of a single species Homo sapiensby postulating gene flow between the evolving groups so that a genetic change in one group would spread to all others and the evolving groups would remain a single species.
His evidence for regional continuity consists mainly of features present in certain fossil material showing either a mixture of Homo erectusand Homo sapienscharacters, or a transition from Homo erectusto Homo sapiensover time.
1. China: As discussed earlier, some of the skeletal remains from China that date to between 100 to 200 kya show characteristics that are interpreted by Wolpoff and by Chinese anthropologists as being intermediate between the Homo erectusfrom China and the modern Chinese (Figure 8). [However, these remains can also be seen as an evolutionary process of Peking Man into a later form that had no evolutionary issue, in a manner similar to the evolution of Homo erectusin Europe into the Neanderthals.]
2. Southeast Asia: The fossils from Australia dating to 30 kya are diverse; some are gracile like Cro Magnon, others are robust and similar to Homo erectus(Java Man). Those that are robust are used by Wolpoff to support a local evolutionary step from Homo erectusto Homo sapiens.The Australian bones show flattening of the frontal bones in a manner similar to that of Java Homo erectus.
3. Europe:
.......a. Some Neanderthals have skeletal features that appear intermediate between Homo erectusand Homo sapiens:reduced cheek musculature, gradual reductions in prognathism. [However, this change occurred only in Europe, not in other regions].
.......b. In the Czech finds at Vindija, late Neanderthals are seen with less robust features. Are their features the result of interbreeding with moderns, or because of their own evolutionary progression? There is not enough material to know whether there was interbreeding. [The fossils seem too late to represent any Neanderthal to modern evolutionary step, since moderns were known in Europe 10 ky or more before these finds.
.......c. Some Cro Magnons have expanded occipital bones as do Neanderthals. [However, this feature is seen in moderns in Africa, Asia, and Australia as well, where there were no Neanderthals].
The alternative to the regional continuity model is the replacement model, in which the populations of Homo erectusthat moved to Europe and to North and South Asia died out without evolving into Homo sapiens.All existing Homo sapiensare the result of the evolution from Homo erectusin Africa, followed by the migration of these people out of Africa less than 100 kya. This hypothesis derives from several authors, most forcefully Chris Stringer and Ian Tattersall.
The evidence for the Replacement Model comes from two sources: the fossils and tools left by humans of 100 to 30 kya, and from the genetics of modern human populations. Neither gives an unambiguous explanation for the pattern, but I think there is reason to favor this theory over the alternative, multiregional theory.
Fossil evidence:
Recall that Homo habilis evolved in Africa into Homo erectussome time about 2 mya. There is no evidence of hominids in any other location at this time, so we can be fairly certain that Homo erectusdid not evolve in Asia, for example, then spread to Africa. Homo erectusin Africa spread out to Asia by 1.6 mya and to Europe by .8 mya. The fossils of Homo erectusin Africa are similar to those in Java and China, though the African bones seem to be less robust than the Asian. All are regarded as one species, though it is uncertain how much gene flow occurred between the African, Asian, and European populations.
First we need to recall what processes make new species. Imagine a group of people living in a land in which the climate is changing (say, from warm to glacial). Within a family, there will be offspring with different traits, and these traits will be heritable. Those individuals with traits that are adaptive in the new environment will survive and breed. Because the environment changes over time, the selected traits will also change, and the population will come to have a different genetic composition anddifferent appearance. When the environment changes, the traits that were adaptive in the previous environment may no longer be adaptive.
This is microevolution,a change in gene frequencies of a population without speciation (the origin of a new species); microevolution is very important to generate local adaptations to a variable environment, but has little to do with the mechanisms by which one species changes into another.
Macroevolutionoccurs by several mechanisms: in one of these, imagine a single interbreeding population. If a geological or environmental event separates this population into two, the divided populations will diverge genetically if the local environments call for different adaptations. After the two populations have been separated for a long time, their genes may become so different that if they are once again reunited, they cannot interbreed.
Another macroevolutionary mechanism occurs without geographical separation. This mechanism involves the difference between two sets of nuclear genes that exist in any organism: the structural genes that encode the adult's enzymes and structural proteins such as muscles; and the regulatory genes that operate only during early development to determine the general body plan of the developing embryo. One regulatory gene affects the action of dozens to hundreds of structural genes by determining whether the structural genes make their proteins or not. It is possible to see that a single mutation in a regulatory gene can have far more important consequences for an organism's structure than a single mutation in a structural gene, because one regulatory gene mutation can completely alter the types of proteins produced and the timing of production of each protein. In addition to making large structural alterations, mutations in regulatory genes can also produce these changes rapidly, over one or a few generations.
Thus changes in regulatory genes are now seen as a major mechanism for macroevolutionary change; there is little doubt that they have operated along with geographical separation to generate the rapid and extensive changes in the human lineage over the past 4 million years.
When a lineage splits and two new species arise from macroevolution, could C and B in (Figure 12) be the same species, or must they be different? Paleontologists would generally regard C and B as separate species, when it is clear what the pattern of evolution has been. This is because the two splitting steps represent separate evolutionary processes, and a single species cannot come from this sort of divergence.
This example seems to apply fairly well to the case of human evolution: The term "Archaic Homo sapiens"is used to denote fossils from the period 500 kya to 100 kya that seem to be ancestral to Neanderthals and to modern humans. However, if Neanderthals and moderns were separate lineages off the Homo erectusline, then not only must Neanderthals be given their own species name, but we must also stop using the term "Archaic Homo sapiens"to refer to people who were ancestral to the Neanderthals. Indeed, the term "Archaic Homo sapiens" used in this way implicitly assumes the correctness of the Multiregional Model, since it places both modern Europeans and Neanderthals in the species Homo sapiens.
Yet though we do not have a complete fossil record for the important period of 500 to 200 kya, the genetic evidence is clear that the lineages leading to moderns and Neanderthals have been separate for a longer time than would be allowed by the Multiregional Model.
.....Neanderthals and modern humans have different appearances that have been distinct as long as both have been seen in the fossil record. Neanderthals originated more than 200 kya and moderns about 100 kya, and the morphological characters of the two groups have been roughly constant throughout that period. They seem to have evolved from separate groups of Homo erectus,one in Africa and the other in Europe.
.....The two groups occurred together in two locations for several thousand years, in Israel 100 kya and in France 35 kya. In neither place is there evidence that the two interbred. They appear to have been reproductively isolated. It is not certain whether the two were actually in the same precise location at the same time, but it is possible because of the Chatelperronian industry, which may be a Neanderthal tool assemblage with Cro Magnon influence. Using this scenario, it is unlikely that gene flow occurred between Neanderthals and moderns in Europe. It is not known whether such interbreeding occurred between the groups of Homo erectusin Asia and those in Europe or Africa. However, the possibility of gene flow between the separate populations of Homo erectusin Africa, Europe, and Asia has been used by some authors as a way of permitting parallel evolution -- the evolution of separate "races" of Homo sapiensfrom separate populations of Homo erectusin Asia, Europe, and Africa.
Genetic evidence
All animals and humans have two kinds of DNA -- that in the nuclei, which is the genetic code for almost all the body's proteins; and that in the mitochondria, which encodes some of the proteins used by these energy-producing organelles. The mitochondria have DNA because at a time in the distant past, they were free-living oxygen-using organisms (a type of bacteria) with their own genomes. They became incorporated into other cells that could not use oxygen, and thereby spawned the eukaryotes with their complex structures and oxygen-using capabilities. Mitochondria are present in all plants, animals, fungi, and protists. Though they are present in all the body's cells, mitochondria in sperm are not transmitted to eggs during fertilization so all an organism's mitochondria (hence all its mtDNA molecules) are inherited from its mother. Both types of DNA (nuclear and mitochondrial) have been used for gathering information about the relationships between groups of modern humans.
Use of modern genes to understand human evolution:Blood samples are drawn from several people of a particular group and genes are obtained from cells that are present in the blood. Or, proteins are studied in the same way. It doesn't matter which we study because genes specify proteins, so the sequence of amino acids in a protein tells us much about the structure of the gene that made it. The proteins or genes present in one human group are compared with those of other groups
For example, you and other members of your family have slightly different gene sequences from those of other people not closely related to you. Imagine three classmates Mary, Frank, and Abdul. Mary and Frank are both from Irish families, but Abdul is from Arabia. Let's say that genetic studies show that Frank and Mary's DNA is 0.04% different, that the two of them differ from the other Europeans in the class by 0.07%, and that these all differ from Abdul by 0.1%. If you make a branching diagram of the differences, you would see these relationships (Figure 13): Mary's and Frank's genes are more similar to each other's than either is to Abdul's, and the figure expresses that relationship. The vertical lines reflect the genetic distance, the time that the families have been distinct. Frank's and Mary's families have a common ancestor (probably an Irish family of some ten or fewer generations ago) that is shown as point c; their family tree also has a common ancestor with everyone else in the class, though it is much earlier, perhaps dozens of generations ago, at point b. Finally, the common ancestor between all of them occurred much earlier yet, perhaps hundreds of generations ago, at point a.
The diagram by itself does not tell us how long the lineages have been distinct; that would have to come from historical evidence (or, in the case of fossils, radiocarbon or some other form of dating). The pattern of relationships shown by all human groups is shown next (Figure 14): This figure shows the same sort of information as in the previous figure, though for all modern humans. It shows three important features: First, all humans come from a single stock; second, the genetic complement of Africans is unique: they are more different from other humans than those other humans are from each other. Third, the rest of modern humans are divided into two main groups, one including Europeans and northeastern Asians, the other including southern Asians.
This analysis indicates that Africa is the source of all human DNA; other human groups split off from this rootstock at some time in the distant past (the first split at the bottom); the second split occurred more recently as some of the original Homo sapiensmoved to southern Asia, and the others to Europe and northern Asia.
These data are consistent with a model that proposes a single African origin for all humans from a group of Homo erectus.They are not consistent with a model that requires Asian, African, and Europeans to have come from separate stocks of Homo erectus.Even with gene flow between populations, the observed pattern would not appear, since the gene flow would make all three groups equally closely related. The only thing missing from the diagram is the time that the splits occurred. This cannot come from the genetic data, but must be gotten by dating the fossils that represent those branching points.
The degree of differences among the nuclear genes of the world's peoples suggests that the original split between Africans and the rest occurred about 120 kya, and that the European/Asian split occurred about 60 kya.
Now let's turn to another way of looking at these relationships:
Mitochondrial DNA:Recall that the mitochondria have their own DNA. A female has trillions of cells, of which a few hundred are eggs. Human eggs also contain mitochondria, and when an egg is fertilized, the new zygote develops with the nuclear genes from the father and the mother, but with mitochondrial DNA only from the mother. Thus all peoples' mitochondria are identical to those of their mothers; brothers and sisters also have mitochondrial DNA identical to each other's and to their mother's. Fathers do not contribute mitochondrial DNA to their children because the mitochondria are lost from the sperm at fertilization.
Over a long time, random mutations will crop up in mitochondrial genes. Thus people alive today have slightly different mitochondrial DNA from their far-back maternal ancestors. The farther back, the greater the difference is. The relationships between different peoples' mtDNA can be studied by
Œ‡ breaking its DNA up into short pieces with enzymes that cleave DNA in special locations;
Œ‡ comparing the DNA sequences of these pieces by gel electrophoresis.
Studies of mtDNA began in the 1970s; the initial work showed that most people had nearly identical mtDNA. This indicates that whatever differences exist, they are of recent origin. Cann, Stoneking, and Wilson wanted to know whether the pattern of mtDNA relationships would be the same as the pattern seen with nuclear genes. So they gathered mitochondrial DNA data from many people around the world and measured the number of mtDNA sequence differences. The comparisons produced a diagram of the same sort as we saw with nuclear genes (Figure 15). The lineage is curved so all subjects in the study could be fit on it; the sequence divergence data suggest that all human mitochondrial DNA has a recent origin, and that the origin is in Africa.
The important question is when the original split occurred.
.......If it happened between .5 and 1 mya, then there is a possibility that it represents the initial Homo erectus migration out of Africa, and the various groups of people seen on the figure are separate evolutionary events from separate populations of Homo erectus in Africa, Asia, and Europe. This would be consistent with the regional continuity model.
.......If the split occurred less than 200 kya, then it is too late for the Regional Continuity Model, because all but a few isolated populations of Homo erectus were extinct by 200 kya or so and because recent evidence (see below) shows that the youngest common ancestor of Neanderthals and modern humans existed more than 500 kya. Instead, it suggests that the a point on the graph represents a group of Homo sapiens,in Africa, some of whom remained in Africa, and the rest of whom migrated out of Africa about 100 kya.
Cann et al. interpreted the data to suggest that the split occurred between 140 kya and 290 kya. They thus favored the replacement model for human evolution rather than the regional continuity model.
Cann et al.'s study generated lots of controversy because of its conclusion in favor of the replacement model, even though nuclear genetic data had already given us the same set of relative relationships among the world's people and had also given a timetable that agreed with the replacement model and not the regional continuity model (Figure 14).
However, it is still instructive to look at the attacks on Cann's work because the conclusive chapter to this story seems to come from another mitochondrial DNA study:
Œ‡ Critics also contend that the computer program used to make the figure could have put the lines in a different order, but the authors chose the order they wanted to emphasize.
Critics were quite vociferous that the replacement model didn't make any sense, because it implied that everywhere in the world, Homo erectussuddenly became extinct after only one of its populations -- in Africa -- evolved to Homo sapiens.It also implied that members of the species Homo sapienswere so warlike that wherever they arrived in the period about 100 to 60 kya they eliminated all the other humans (e.g. Neanderthals in Europe, perhaps some archaic groups in Africa and Asia) without any interbreeding. Instead, the critics maintained that Homo erectusevolved in northern Asia into the Chinese; in southern Asia into the Australians and southeast Asians; in Europe the Neanderthals evolved into Europeans.
The argument might have stalled at this point for some time but for a new mtDNA study from American and German labs in the summer of 1997. In this work by Krings et al., mtDNA was extracted from a small piece of the humerus from the Feldhofer Cave Neanderthal discovered near Dusseldorf in 1856 (the specimen that gave the name to this group of people). Its DNA sequence was more similar to that of modern humans than either was to chimp mtDNA, indicating its undoubted human affinities. But the Neanderthal sequence was outside the range of variation of modern human mtDNA sequences: whereas modern populations show about 8 base-pair differences in that particular portion of the mt DNA sequence studied, the Neanderthal DNA showed an average divergence from modern sequences of 27 base pairs (Figure 16).
The mtDNA of the Neanderthal showed no closer relationship to European DNA than to that of any modern human group. The researchers also computed from this number of differences that modern human and Neanderthal sequences diverged at least 500 kya. These two features -- the long amount of time and the lack of close resemblance to European DNA -- are not consistent with an origin of modern Europeans from Neanderthals, but are consistent with the Replacement model, in which Neanderthals are seen as an early offshoot of Homo erectus that did not lead to later groups of people.
Another study of Neanderthal mtDNA was recently published by Ovchinnikov et al.. They isolated mtDNA from the skeleton of an infant found at Mezmaiskaya, Caucasus Mountains (29 kya). The baby was either a near-term fetus or a newborn infant, and a sample of DNA indicated that the nucleotide sequence was more similar to that of the Feldhofer Cave Neanderthal than to modern humans. As in Krings' study, the sequence homology suggests that the most recent common ancestor of Neanderthals and modern humans lived about 500 kya. Thus the two existing Neanderthal mtDNA sequences give the same picture of Neanderthal relationships to modern humans.
Adcock et al. sequenced mtDNA from several Australian skeletons dating from 2 kya to 60 kya. Most, including the oldest (Lake Mungo 3) were anatomically modern; those from Kow Swamp are often described as possessing "archaic" traits linking them anatomically to Homo erectus (Jurmain et al.). The mtDNA from all the skeletons fell within the range of modern human variability.Finally, Caramelli et al. compared the mtDNA of extant modern Europeans, extant modern non-Europeans, fossil anatomically modern humans dating between 23 and 60 kya, and four fossil Neanderthals dating between 29 and 42 kya. The data are shown in Figure 17. The graph represents a Multidimensional Scaling analysis (MDS) in which genetic distance is shown by the linear distance between two points in both graph dimensions. The triangles are modern Europeans, the closed circles are modern non-Europeans, the open circle is an Australian skeleton from Lake Mungo (60 kya), and the open squares are two italian skeletons (23-25 kya). The diamonds are from four Neanderthal skeletons. The closed circles and triangles show that extant moderns, whether European or not, have similar mtDNA sequences. The open circle and the open squares show that anatomically modern skeletons of age 23-60 kya are within the same range of variation as extant moderns; the diamonds indicate that the Neanderthals of the same age are outside the range of variation of of moderns. These data are consistent with the Replacement model in suggesting that Neanderthals are not more closely related to Europeans than to any other extant humans.
Throughout the history of this controversy, interpretations based on DNA evidence have been at odds with interpretations based on skeletal evidence. This conflict has been exacerbated with the recent discovery of a skeleton in Portugal (at Lagar Velho) of an Upper Paleolithic burial dating to 24.5 kya. The nearly complete remains of a child, estimated at 4 years of age, was found buried with shells and ochre in the Gravettian tradition (before seen only with anatomically-modern humans). The discoverers, Duarte et al. suggested that the skeleton has features that resemble both Neanderthals and moderns: features of the skull and jaw appear to be modern, but the proportions of the limbs resemble Neanderthals rather than moderns. They argue that the constellation of characters in the child resembles known hybrids in modern non-primate mammals. Because of the late date (5,000 years after the last-known Neanderthal site in southern France), the authors suggest that the child was not a Neanderthal-modern hybrid, but rather the result of generations of interbreeding between the two groups; this indicates to the authors that the two groups are members of the same species and that the Replacement Theory is therefore incorrect.
In the same issue of the journal in which Duarte et al. Published their findings, Tattersall and Schwartz give a commentary on Duarte et al.'s interpretations of the Lagar Velho child. They disagree that the skeletal remains suggest a long period of interbreeding between Neanderthals and moderns, because even though the first generation of hybrids in other mammals exhibits characters found in the two parental lines, continued interbreeding for several hundred generations (the time between 29 and 24 kya) should minimize the distinct contributions of the parental lines, and the child should not have possessed so many "dichotomous" characters -- those that are either modern or Neanderthal. They conclude that the skeleton is that of a "chunky" or "heavily-built" modern human that does not show admixture of modern and Neanderthal genes.
Of course the story could not end there. Trinkaus and Zilhao, two of the authors on the Duarte paper, gave an extensive and sometimes emotional reply to Tattersall and Schwartz (this reply is no longer available on the web). The crux of their response is their view that even after 100-200 generations of interbreeding, dichotomous characters would still be preserved in distinct form if they were adaptive as individual characters.
It is not clear how this controversy will play out. It is obvious that mtDNA from the Lagar Velho child will be important to obtain, because the interpretation of Duarte et al. is based only on skeletal features. However, it is difficult to know what to do with mtDNA in this case, since the mitochondria are only maternally inherited. It is possible that even if the child is a Neanderthal-modern hybrid, the mtDNA will have a sequence characteristic only of the mother's line. If that is Neanderthal, it will resemble that of the Feldhofer Cave Neanderthal; if the mother was modern, the mtDNA will be closer to that of modern humans. However, since no examples of mtDNA have yet been obtained from anatomically-modern humans of the Upper Paleolithic, we do not know how much resemblance there is between people of that period and today. Therefore, even if the Lagar Velho child is the product of 100-200 generations of interbreeding, his or her mtDNA would likely show either a modern or a Neanderthal profile, but very likely not one in between. When the mtDNA of the child is eventually sequenced, only one of two possible outcomes will allow a conclusive interpretation: if it resembles moderns, opponents of the Replacement Theory will not give up because they will be able to state that the paternal Neanderthal genes were lost. Only if the mtDNA pattern is fully Neanderthal will the evidence suggest that the child resulted from a mixture of the two groups.
This still does not answer the question of what the Lagar Velho child is. If only the part of the body above the lower limbs had been found, it is likely that it would have been classed as modern on the basis of cultural artifacts and cranial/upper limb structure. However, even though Duarte et al. and Trinkaus and Zilhao state that the lower limb proportions are closer to Neanderthals than to moderns, there are still questions: Could local climatic conditions of late-glacial Portugal have selected anatomically-modern humans for stocky build? Do we have enough examples of late-glacial human skeletal material to state what the range of variation is among all the groups of people living in Europe at the time?
The most important obstacle in the way of understanding these relationships is the inconsistency of the evidence from skeletal and genetic sources. On the one hand, we have mainly problematic bones that could be used to support either the hypothesis that Neanderthals and moderns are distinct or that they may have interbred; on the other hand we have mainly non-problematic genetic evidence from modern humans and from Neanderthal bones that clearly supports only the hypothesis that moderns and Neanderthals have been genetically isolated for several hundred thousand years. These data suggest that the two groups belong to different breeding species, but say nothing concerning their membership in one or more morphological species. The controversy thus reminds us that there are two (sometimes inconsistent) ways of defining species: by morphology or by reproductive isolation. In my view, if in a particular instance we need to choose between the two, reproductive isolation is the easier to infer. This is because morphological similarities can represent parallel or convergent evolutionary pathways and thus can be deceptive of true relationships. Only a more complete genetic and structural assemblage of late Pleistocene humans will give us enough evidence to make clearer judgments.
What happened to the Neanderthals? Whether they were conspecifics with modern humans or not, their unique anatomy and gene structure no longer exists in the form seen in the fossil record (though adherents of the Multiregional Theory claim that their genes survive in the genomes of modern humans). As stated above, Multiregionalists doubt that modern humans would have exterminated the Neanderthals by warfare, and perhaps that is not needed as an explanation for their disapperarance. All that would have been required is that they have a higher mortality rate. Zubrow has calculated that a mortality rate 2% higher than that of modern humans of the same period would be more than adequate to explain their observed extinction pattern shortly after the arrival in Europe of modern humans.
In the past few years, geneticists have accelerated their studies of mtDNA for the purpose of identifying migration patterns of Paleolithic and Neolithic peoples. Because of these studies, we have more sequence data from more populations than were available when the Multiregional and Replacement theories originated. The evidence gives a clearer idea of the migration routes followed by many groups of people over the past 150 kya, and appears to be consistent with the Replacement view of a recent African Origin followed by migrations out of Africa to Asia and Europe.
For example, in a recent study, Maca-Meyer et al. describe the history of human mtDNA lineages; their data (some of what follows is shown in Figure 18) allow the following conclusions:
.....1. Current human mtDNA lineages originated in Africa somewhere between 156 and 169 kya;
.....2. Two major early divergences within Africa date to 122-132 kya and 85-95 kya;
.....3. Two lineages moved out of Africa 59-69 kya Ü- one to south Asia and one to north Asia [followed by splits to Northeast Asia (53 kya) and to Europe (39 kya)].
If the Multiregional theory were correct as proposed, it would not be possible for mtDNA evidence so consistently to indicate the overall unity of the human genome along with such clear evidence of recent movements of the people bearing these genes. Instead, all extant human groups would show similar degrees of variability and lineages would have a more diffuse distribution.
When one looks at all the data (fossil, nuclear DNA, mtDNA), it is most likely that something close to the replacement model reflects the pattern of recent human evolution:
1. The Neanderthals in Europe and the Near East are unlikely to have been in the same species as the modern Europeans because the two groups were neighbors for 40 ky with little (Trinkaus) or no (Tattersall) evidence of interbreeding. Neanderthals seem to have evolved from Homo erectusin Europe, then lived there for 200 ky until the moderns arrived. Homo sapiensseems to have originated much later from other Homo erectusin Africa. The replacement of Neanderthals by moderns occurred with no clear evidence of an evolutionary step -- the Neanderthal structure and culture just died out.
2. Homo sapiensfossils are seen earliest in Africa at 130 kya (though Chinese anthropologists claim that Homo sapiensexisted in China by 200 kya. This Chinese material resembles the "archaic" finds from Europe rather than Homo sapiensas we know it now.
3. The genetic data (nuclear and mtDNA) suggest that all modern humans are closely related, and that genes have diverged from a single source rather than come from parallel sources. These data also suggest a recent African origin for our genes.
4. The Neanderthals existed with little structural change in Europe for 200 ky. Their anatomy suggests robust structure, different use of muscles than moderns, different laryngeal structure that may have made speech of the type seen in modern humans unlikely.
5. Another type of data comes from the cultural remains of the Neanderthals and the moderns.
.......a. There is no art (painting or carved stone) from any Neanderthal site, though Cro Magnons contemporary with the late Neanderthals had both.
.......b. The tools of the Cro Magnons were much more complex, involved more working and more planning, and produced more useful surface per core than that of the Neanderthals.
.......c. The scale and complexity of Neanderthal settlements was smaller than in the contemporary Cro Magnons: Neanderthals gathered stone for tools from closer to their camps, they were more self-sufficient and had less trading with other groups. The Cro Magnons gathered materials from a much wider region and clearly traded with other far-away groups.
Figure 19 gives a summary for this paper, and an overview of an evolutionary pattern for modern humans that is consistent with the geological and genetic data.
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Updated July 2005
