Human Evolutionary Tree

The study of human genetics has added a new layer of evidence to the story of how human beings evolved and migrated on Earth.

Genetically, humans are far more similar than they are different; moreover, evolutionary changes in the genome itself are hard to tease apart, having been shuffled and reshuffled so many times. However, two mechanisms of genetic inheritance—one paternal and one maternal—do not involve recombination and have proved to be valuable tools for understanding the relationships among human beings from different continents and from different times in history.

As previously mentioned, much of what we know about the evolution of the human genome comes from our knowledge of two specific mechanisms of inheritance. The first mechanism involves the transmission of certain paternal genes. Specifically, scientists have shown that there is little recombination between the X and Y chromosomes; thus, most of the markers on the Y chromosome are transmitted from father to son over many generations. In contrast, on the second or maternal front, mitochondrial genes are passed from mothers to children of both sexes by virtue of the fact that the mitochondria contributed to the embryo by the egg far outnumber those contributed by the sperm. Of course, both Y chromosome genes and mitochondrial DNA (mtDNA) are subject to mutation; furthermore, these mutations occur at two to three times the rate of autosomal mutations (Jorde et al., 2000). Tracking such mutations thus allows researchers to follow distinct lineages. In fact, one region of particular importance in mtDNA is the hypervariable region (HVR1 and 2), where the rate of mutation has been shown to be up to 100 times greater than that of the nuclear genome (Sigurgardottir et al., 2000).

One of the seminal papers examining the use of mtDNA to track human lineage was published in 1987. In this paper, Cann et al. considered patterns in human mtDNA samples from five different geographical regions that spanned the globe. By examining certain restriction site polymorphisms in these samples, Cann and colleagues identified 133 mitochondrial haplotypes. Using this information, the researchers concluded that the evolutionary tree of mtDNA has its roots in Africa, and that the branches of this tree are short enough to hypothesize that all humans have a fairly recent common ancestor (sometimes called "mitochondrial Eve").

Indeed, many human subpopulations carry distinct markers, and by tracing these markers through the generations, one can draw a genetic tree on which today's many diverse branches can be followed to their common African root. Thus, the surprising thing about Cann et al.'s results is not that all people have a common root, but rather how recent this root appears to be. Specifically, Cann and colleagues hypothesized that the ancestress of all of modern humanity lived only 200,000 years ago, rather than more than a million years ago, when primitive humans first walked the earth. According to Cann et al., humans who lived elsewhere in the world at the time of mitochondrial Eve were completely replaced by her descendants - modern-form humans - migrating out of Africa 100,000 years ago.

The out-of-Africa replacement hypothesis has been hotly debated. The main alternative theory is a multiregional hypothesis, credited to Australian fossil researcher Milford Wolpoff, who says that human populations in both Africa and Eurasia contributed to the evolution of anatomically modern humans (Figure 1; Wolpoff et al., 2000).

Just as scientists have used mtDNA to determine the relatedness of all humans, they have also used this genetic material to help determine just how different various groups of people are from one another. Modern genetic research has shown that traditional racial groups have little meaning at the genetic level (Lewontin, 1972). Rather, most of the variation in genetic polymorphisms is related to differences among individuals within populations—some 85% worth. Only about half of the remaining 15%—that which might be used to differentiate subpopulations—actually distinguishes traditional racial groups. This apportionment of variation has been confirmed by recent studies using more sensitive DNA markers (Templeton, 1997). Clearly, far more variation is found among individuals than among groups.

To track the evolution of groups, anthropological geneticists study extremely polymorphic alleles. In addition to mtDNA, human leukocyte antigen (HLA) and short tandem repeat (microsatellite) loci—nucleotide repeats, such as CACACACA—are found throughout mammalian genomes, and they have proved to be immensely useful for inferring the history of human migration.

Still, the interpretation of our genetic archives is a difficult science. One conundrum is trying to tease apart the influence of time and population size. You might think that the longer two populations have been isolated from each other, the more divergence you would see between them. However, in addition to mutation rate, population size can affect diversity. For example, modern Africans have greater genetic diversity than Asians or Europeans—but is this diversity due to their evolutionary age or their greater population numbers during the last million years? One study showed that the diversity of mtDNA in African farming populations was greater than that in Africa's less numerous hunter-gatherer populations (Watson et al., 1996). Because no one believes agricultural peoples preceded hunters, this finding attests to the idea that population size is a critical factor in measures of genetic diversity.

Enter the trellis model put forth by Alan Templeton (Figure 2). This model accepts an African origin of the human species, but it argues that this origin predates a hypothetical mitochondrial Eve, occurring approximately 1.7 million years ago. After that, multiple migrations out of Africa occurred, but the human race was never completely replaced by a single tribe. Moreover, once the three major populations (African, Asian, and European) branched out, they did not evolve in isolation. Rather, periodic movement of subpopulations allowed some genes to flow among the groups. This movement of genes accounts for Templeton's trellis design, which features lateral connections between vertical evolutionary lineages.

Templeton himself writes that the trellis model "posits the Homo erectus population not only had the ability to move out of Africa, but also back in, resulting in recurrent genetic interchange among Old World human populations. Here, there is no split of humanity into evolutionary sublineages, and the human race cannot be portrayed as branches on an evolutionary tree; rather, the genetic closeness or distance of various human populations reflect their amount of genetic interchange and not their time of divergence from a common ancestral population. Under the trellis model, anatomically modern traits could evolve anywhere in the range of Homo erectus (which includes Africa) and subsequently spread throughout all of humanity by selection and gene flow."