Lecture 25: Human Evolution and Diversity
Contents
Lecture 25: Human Evolution and Diversity¶
Evolution of Modern Humans¶
Humans are members of the order Primates which consists of about 180 species (there are 17 different orders of mammals which diverged 80-65 million years ago). Primates are a relatively old order of mammals. Most of the synapomorphies of this order are associated with an arboreal way of life: flexible digits, forward facing eyes, vision as a primary sense. These traits may have played a role in the evolution of brain size in the lineage leading to humans. Humans are a member of the family Hominidae which is believed to have diverged about 5 million years before the present (MYA) from the other members of the Old world monkeys. At least 20 MYA the Hominoids split off from the other old world monkeys. The dates are rough and get changed now and then.
Relationship of humans to African apes (= chimps, gorillas) and orangutan DNA hybridization indicates that apes are our closest relatives. Human/chimp/gorilla relationships not proven but chimps are most likely our closest relatives. The molecular clock says 5 million years ago the human-chimp line split.
While Chimp and gorilla have knuckle walking , the humans posses many traits associated with bipedality: vertebral column, shape of pelvis, angle of femur, foramen magnum at base of skull. Bipedality seems to be a major “innovation” which allowed humans to enter a new “adaptive zone”. The first earliest ancestor in the lineage leading to humans (Australopithecus afarensis) seems to have an angle between the femur and tibia (Upper and lower leg) that is intermediate to that of humans and gorillas.
The evolution of modern humans from our hominid ancestor is commonly considered as having involved four major steps: evolving terrestriality, bipedalism, a large brain (encephalization) and civilization. There are (and have been) several competing hypotheses that have acknowledged these four steps, but put them in a different sequence during human evolution.
Origin of Homo sapiens: Australopithecus afarensis = first bipedal hominid, found in east Africa about 3.0-3.2 MYA. Later forms became more slender (= “gracile”). Homo habilis and H. erectus ( 1.5MYA) came later. The evolution of bipedalism may have freed the hands for us in other functions: carrying, tool use. The trends in the evolution of tool use (more types, more specific tasks, different types of materials, more efficient use of materials) seems to follow (lead??) the evolution of increased cranial capacity. These both seem to increase noticeably about 2 MYA. One theme that involves each of the different sequences of evolution is that there was some feedback that lead to the increase in cranial capacity, e.g., becoming bipedal creates selection pressure for a more elaborate brain to control motor function and to process incoming sensory information. This in turn would allow for more successful bipedalism, etc. The same argument could be leveled about culture leading to an increase in brain size, and vice versa, so the sequence cannot be resolved just on the logic of feedback loops alone.
Origin of “modern humans”: Two alternative scenarios for origins: 1) humans originated in more than one site (“Multiregional” model). Evidence supporting this are modern Homo sapiens samples found in Asia and Africa 2) a single origin (“Noah’s Ark” model: one origin and dispersal out from site of origin). Homo sapiens are believed to have originated 100,000 - 200,000 years ago.
Paleontological evidence suggests a single origin in Africa. Molecular data shows low genetic diversity worldwide with the highest diversity in Africa, aslo suggsting an African origin. Recent re-analyses shown that the cladograms of mtDNA cannot support an African origin on statistical grounds. Moreover, some recent fossil finds have put proto-humans outside Africa about 2.4 MYA, but these may be due to early migrations. However, three independent, relatively-recent articles in Nature (March 31, 1994; vol. 368, pgs. 449-457) all support an African origin for humans; two are based on fossil analyses and one is based on DNA analyses of microsatellites.
The analysis of the evolution of culture and civilization in humans clearly must be based in materials other than human bones alone. The evolution of tools is one reliable correlate (they are recognizable as being rocks reworked as tools and, being rocks, they preserve well). The patterns of tool form show some suggestive trends regarding civilization: through time more types of tools become apparent and there is less variation among specimens in the shape/form of a given tool. This has been interpreted as evidence for communication or “training”, since ‘word may have spread’ on just how to improve that stone ax so that it can be used more effectively for certain tasks.
The spread of Homo out of Africa is presumed to have taken place about 1.5 MYA by Homo erectus. This species seems to be on a trajectory of brain size and body size that looks anagenetic, whereas one lineage that lead to Australopithecus robustus seems to be on another line. In a broad sweep of time, the notion of the chimp leading to the Australopithecine, to Homo, to the Neanderthal to the modern American family standing in their driveway is a myth. There were lineages that diverged in a branching cladogram, some of which did not make it to the present. Evidence for this is provided by more than one distinct morphological type of early humans present at the same time. As time gets closer to modern humans, however (Homo erectus on up), a phyletic gradualist anagenesis is more easy to accept.
Once a big brain is achieved and this provides the intellect for an organism to anticipate its environment, the notion that an organism evolves in response to changes of the environment becomes too simplistic. Humans evolved the power to alter their environment so as to protect themselves from its abiotic pressures. This means that they are altering their own selective pressures and a dialectic emerges between the organism and the environment such that these cannot be separated. Other organisms do this (beaver dams, deciduous trees), but in humans this cycle is accelerating. The rest is history.
Human Diversity¶
Modes of evolution Throughout course we have stressed that traits must have a genetic basis
WHY? for transmission to next generation. What do humans have that allows transmission to next generation without genetics?? CULTURAL transmission. This is a major distinction with other animals (although other animals do have “culture”, it is not as “advanced” as humans) Cultural transmission can be vertical (between generations) or horizontal (within generation). Vertical cultural transmission is Lamarkian: what you acquire during your lifetime you can pass on to your offspring (“inheritance of acquired characteristics”). Some analogies with other evolutionary forces:
mutation: innovation, new behavior, bungee jumping, country music, pierced tongues
selection: popularity, status: trait gets swept into high frequency
drift: random variation in culture, language, dialects (southerners, Maine hicks, etc.)
gene flow: Brah! My cousin in California pierced her tongue!
Communication, and especially language, are further “key innovations” that may have lead humans into a new “adaptive zone”. But literature (Lascaux cave paintings, National Inquirer; and now consider electronic transfer of literary info, e-mail, etc.) really sets us apart. Our rate of evolution is dramatic on all counts. While the beaver, in building its dam, alters the environment that surrounds it, it has evolved in this context of altering its environment in a predictable way for a long time. We are no where near “equilibrium” with respect to how we are evolving with the extensive alterations we are making to our environment. One hopes that we have the genetic wherewithal to “run fast enough just to stay in place”.
All organisms vary and humans are quite good at recognizing nodes or clusters in that variation (demes, populations, species). The nodes or clusters within human phenotypic variation are quite pronounced and most of us would sort the ambassadors to the UN into more-or-less the same “groups”. The question thus arises: how is the genetic variation within humans partitioned? This question was asked (and answered) by R. C. Lewontin in 1972 (The apportionment of human diversity. Evolutionary Biology vol. 6: pp. 381-398). Lewontin collected data on the frequencies of different alleles at various nuclear loci in different human populations. These populations were nested into larger groups we know as races, and the various races can be nested further into the larger group we call the species Homo sapiens.
With data on allele frequencies in each population, Lewontin could ask how much additional variation is added to the unit in question by pooling together the data from all populations within races, or at one level up the hierarchy, by pooling all the data from within races to one large species sample.
To quantify what proportion of the total genetic variation within humans, a measure of “heterozygosity” was used similar to \(H = 2pq\) (or \(1-p_i^2\) for i different alleles; in the paper the very similar Shannon-Weaver index was used). The numbers obtained from this formula provide a measure of genetic diversity at each of the many loci tabulated by Lewontin. Four points are relevant in thinking about this measure of “diversity”: 1) a locus with only one allele will have \(H = 0\); 2) the greatest diversity will be when all alleles are equally frequent (\(p=q=0.5\) for 2 alleles; \(p1=p2=p3=p4=0.25\) for 4 alleles); 3) diversity will increase as the number of alleles increases (the 4 allele case above is more ‘diverse’ than the 2 allele case); 4) diversity is a convex function of allele frequency (a diversity measure from a pooled sample obtained by combining alleles from two different populations will be greater than the average of the two diversity measures from each population).
These H values can be tabulated for several different levels of the hierarchy described above: by considering a single population’s \(p\) and \(q\), or the average \(p\) and average \(q\) within a single race (averaged among all populations within that race = \(H_{\mbox{pop}}\)), or the average \(p\) and average \(q\) for all races (pooling all populations within a race to determine the p for that race, then averaging the \(p\)s for each of the races = \(H_{\mbox{race}}\)). \(H_{\mbox{species}}\) will be determined by pooling all populations irrespective of race to obtain a species-wide \(p\), then calculating \(H = 2pq\) (or as above for the multiple allele case). Thus we can have an \(H_{\mbox{pop}}\) which will be less than \(H_{\mbox{race}}\) which, in turn, will be less than \(H_{\mbox{species}}\).
These \(H\) values can then be partitioned or apportioned so that the total variation within humans can be attributed to the within population component or to the within race component or to the between race component. The simple expressions for these, and the data for each locus are presented below. The conclusions from the results are very clear. Of all the variation within humans, 85.4% of it lies within populations (i.e. is due to variation among individuals within populations). An additional 8.3% lies between populations within races. Only 6.3% of all the genetic variation within humans is due to differences between races!
Lewontin concludes that there is no genetic or taxonomic basis to racial distinction and classifications of this sort are of no social value. Recent analyses with microsatellites in human populations give slightly different numbers, but the general conclusions are the same. While you are free to agree or disagree with Lewontin’s social interpretation of the data, the population genetic conclusions are clear: with the largest component due to variation among individuals within populations, each and every one of us matters.