Paraminder Dhillon

Humans, A migration out of Africa?

Humans are one of the most formidable species on Earth. Our greater intellect and ability to reason sets us apart from other members of the animal kingdom and has allowed us to exploit habitats and resources worldwide. So why, in the age of mankind, is there only one surviving species within the Homo genus? How have modern Homo sapiens come to reign over their archaic forebears, in the course of hominid evolution?

Despite being a topic that has dominated evolutionary studies over the

last two decades, the origin of modern humans remains enigmatic. Most

theories of human evolution are variants of two conflicting models - the

"multiregional" model and the "out-of-Africa replacement"

(OAR) model. Both explanations accept Africa as the evolutionary origin

of the Homo genus and the subsequent migration of these early hominids,

Homo erectus, into Europe and Asia around two million years ago. However,

the models differ in their account of H. sapiens evolution, which is where

the controversy lies. Was Africa the sole place of origin of our own species,s

as well as that of our ancestors?

Figure 1 : Evolutionary models - (a) Multiregional model: Modern humans arise simultaneously in the three populations over time, due to gene flow and genetic continuity (b) OAR model: Lineages in Europe and Asia are supplanted by the African lineage, with little or no hybridisation between groups.

The multiregional model proposes that modern humans evolved simultaneously

in different regions of the Old World from populations of archaic humans

(Homo erectus). A single, dominant species, Homo sapiens, is the outcome

of this "parallel evolution" as gene flow between populations

is possible (figure 1a). Under this model, specific regional characteristics

are retained in each geographically dispersed population of modern humans.

The out-of-Africa (OAR) model, on the other hand, emphasises replacement

as a means of eliciting the spread of Homo sapiens. Proponents of this

view hold that a single biological population of anatomically modern humans

initially evolved in Africa at some time over the last 100 - 200 thousand

years, and then wandered out of Africa, driving the pre-existing hominid

populations to extinction (figure 2). Amongst these ancient populations

were the Neanderthals of western Eurasia and immediate descendants of

Homo erectus, which are thought to have made no genetic contribution to

modern humanity (figure 1b). This theory is now thought to be the most

likely, based on a growing body of archaeological and, more recently,

genetic evidence.

THE CASE AGAINST THE MULTIREGIONAL MODEL

A crucial element of the multiregional model is gene flow between

modern and archaic humans, which would facilitate synchronous evolution

into a single species. However, this fundamental aspect has been challenged

primarily due to the lack of transitional fossils in Europe and Asia,

which would indicate that such gene flow was taking place. The absence

of interbreeding is further confirmed by molecular evidence, such as recent

studies of ancient Neanderthal DNA. According to the multiregional theory,

Neanderthal DNA should resemble that of Europeans more than the DNA of

modern Africans and Asians since Neanderthals (figure 3) were known

to have lived in Europe and Western Asia from around 230 - 30 thousand

years ago. But in reality, Neanderthal DNA seems to be distinct from all

modern human DNA, regardless of race and geography. The extent of genetic

difference between the two supports the view that Neanderthals coexisted

as a separate species that has gradually been supplanted by modern humans,

without contributing to the modern human gene pool.

Figure 2: Journey of humankind - modern humans originated in sub-Saharan Africa and dispersed within the last 100,000 years (as outlined in the OAR model).

Molecular studies also emphasise the unique importance of Africa. Evidence

suggests that Africans exhibit a higher level of genetic diversity than

non-Africans for many mitochondrial and nuclear loci. This implies that

the African line has been around for longer than other populations, enabling

greater genetic variation to accumulate, which is in agreement with the

OAR model.

Figure 3: Driven to extinction - Neanderthals in Western Eurasia

MITOCHONDRIAL EVE

Nevertheless, the most compelling evidence to date comes from mitochondrial

DNA (mtDNA) studies. If modern mankind has evolved from a subpopulation

of humans originating in Africa, the notorious figure somewhat misleadingly

termed Mitochondrial Eve, may have been amongst these early humans.

But who is this elusive female and how has she helped us gradually to

unravel our genealogical history?

Figure 4: A lucky horseshoe for proponents of the OAR model - Genealogical tree for different types of human mtDNA. The black arrow points out the two primary branches, one of which leads exclusively to African individuals, indicated by red circles. The other branch contains all other racial groups as well as Africans. Thus we can infer that the common ancestor ("Mitochondrial Eve") was of African origin.

Contrary to common misconceptions, Eve was not the sole living

female of her time and therefore not the only female ancestor of modern

humans. However, she is our most recent common ancestor with respect to

matrilineal descent. Mitochondria, the powerhouses of our cells, provide

an insight into the female genetic history, as unlike nuclear DNA, mtDNA

is not a hotchpotch of sequences derived from both parents but is inherited

solely via the maternal lineage. In theory, the mitochondria in the cells

of all modern humans could be traced back to a single woman - nicknamed

Mitochondrial Eve. Although other females were present during Eve's

time, by pure chance, she was the only woman to produce a continuous

line of daughters that persists today, reflected in the preservation of

her mitochondrial genome.

Due to its rapid mutation rate and uniparental mode of inheritance, mtDNA

is a particularly useful tool in evolutionary studies. Under the assumption

that any mitochondrial genome diversity is due to mutations alone, and

that these random mutations occur at a roughly constant rate, sequence

data can be used to approximate when two populations last shared a common

ancestor. In this way, phylogenetic trees can be produced.

The pioneering mtDNA studies focused on a control region of the genome

termed the D-loop, which has the highest mutational rate. This region

was sequenced from individuals distributed around the world, and annual

mutation rates estimated, enabling scientists to wind-back our molecular

clock to the time of the founder genotype from which all the mitochondrial

genotypes in modern humans arose. This technique predicts that Eve

lived approximately 150,000 years ago, and was an African female (figure

4).

However, more recent studies have concentrated on the entire mitochondrial

genome. The complete genomes of several people from different geographical

and racial backgrounds have now been sequenced, yielding evolutionary

trees with deep indigenous African roots. As the statistical support for

these phylogenies is strong, an African origin for the human mitochondrial

gene pool is highly convincing. The ancestral mtDNA from which all modern

mtDNAs are derived is thought to have existed in an African population

approximately 170,000 years ago, which is consistent with the notion that

anatomically modern humans evolved in Africa within the last 200 thousand

years. Mitochondrial Eve probably predated the exodus out of Africa,

and her mtDNA was amongst the founding population that came to displace

archaic populations.

Mitochondrial DNA may provide a picture of maternal genetic history,

but as only one genetic locus this is not enough to substantiate an entire

theory of human origin. Eve's genetic complement, however, Y-Chromosome

Adam corroborates earlier findings. Analyses of Y-chromosome sequences

have allowed researchers to uncover the genetic trail leading to an Adam

with unmistakeable African roots.

THE FUTURE

So, as the evidence mounts in favour of a recent African origin, one

might ask why we continue to speculate about our evolutionary history.

Why are we still digging if the roots have been unearthed? The answer

is that in spite of the facts, there is still no final answer. None of

the deductions made thus far are watertight, and the methods and approaches

employed are continually being reassessed. For instance, over recent years

the assumption that mitochondrial DNA is maternally inherited and thereby

free from recombination has been disputed. If sperm mitochondria are found

to recombine with mitochondria present in the ovum, the credibility of

the mitochondrial evidence may be called into question. Similarly, flaws

in the molecular clock technique have been highlighted.

The story is further complicated by the possibility that neither of the

principal models (OAR and Multiregional) is correct. The true explanation

may be an amalgamation of the two, which is reflected in the alternate

"Hybridisation" and "Assimilation" models. These theories

tone down the role of replacement in human evolution by incorporating

gene flow and hybridisation yet still allow that Africa has a prime position

in human genetic history. The exact importance of Africa, and indeed the

full narrative, remains to be told. But with further advances in molecular

techniques, and the use of alternate gene systems, we may finally be getting

closer to solving the mystery of where we came from...

ACKNOWLEDGEMENTS

- The image of the Neanderthal skull was kindly provided by Ulf Gyllensten

of the University of Uppsala, Sweden

- Figure 1 was adapted from Philosophical translations: Biological Sciences

Vol. 357, No. 1420, 565 (Royal Society 2002)

- Figure 2 courtesy of www.nature.com (Nature 408, 7 Dec 2000, p. 653)

- The mtDNA tree was adapted from Scientific American, 266/4, 69 (April

1992)

- February 2006

About the Author

Paraminder Dhillon is studying biochemistry and genetics at Nottingham University.