How Have Humans Populated the Earth - Pavel A. Pevzner

? Phillip Compeau & Pavel Pevzner 2017. All rights reserved.

How Have Humans Populated the Earth?

Out of Africa

An evolutionary quandary in Darwin's writings

Charles Darwin's 1859 On the Origin of Species by Natural Selection suffers from a glaring omission. If natural selection is a universal phenomenon, and humans are undoubtedly a part of nature, how then have we evolved? Darwin did opine on the matter, but only sparingly:

"In the distant future I see open fields for far more important researches. Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation. Light will be thrown on the origin of man and his history."

When Carl Linnaeus published the tenth edition of his famous taxonomy a century earlier, he had been so bold as to combine humans and monkeys into a single order, "Primates". Our old friend Comte de Buffon from Chapter 2 even stated in 1766 that in terms of anatomy, the orangutan

"is only an animal, but a very singular animal, which man cannot view without returning to himself".

Nevertheless, the concept that we share a common ancestor with monkeys was a far more radical one than the idea that natural selection had created giant turtles in the Galapagos. Darwin himself remained silent on the issue, since there were no clear fossil records linking us to our evolutionary predecessors. Little did he know that the answer to the puzzle of our origins was already dawning across the English Channel.

The fossil record elucidates human evolutionary history

Three years earlier, miners working in a German cave had uncovered heavily calcified skeletal remains. The excavators initially thought that the bones belonged to extinct cave bears, which we mentioned in Chapter 11 as being extremely common in Europe until the most recent ice age. Yet some researchers suspected that these fossils belonged to a member of an early human population. The name that they bestowed upon these ancient humans was borrowed from the valley in which the remains had been excavated: Neanderthals.

In the ensuing years, the fossil record accumulated evidence in favor of our evolution several million years ago from a chimp-human ancestor. Throughout the 20th Century,

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? Phillip Compeau & Pavel Pevzner 2017. All rights reserved.

scientists discovered fossils from intermediate species between chimps and Neanderthals like australopithecines, Homo habilis, and Homo erectus (Figure 1).

Figure 1: A timeline of recent human ancestors. All these species have only been discovered in Africa until Homo erectus, whose fossils have been found across Europe and Asia. Neanderthals arose in Europe and are often considered distinct from Homo sapiens, a species that includes Cro-Magnons. Courtesy: Wikipedia. Until 2 million years ago, all our previous ancestors had been confined to Africa. Yet researchers have discovered Homo erectus fossils throughout Eurasia. We can therefore conclude that Homo erectus must have been the first human ancestor to emigrate from Africa and survive long enough for us to be able to find their fossils. Genetic data resolves the origin of modern humans What happened next became the subject of a much larger debate. Monogenists held that despite the far-flung travels of Homo erectus, modern humans arose recently in only one location. Polygenists, on the other hand, believed that modern humans evolved from Homo erectus along distinct paths in different corners of the globe. The polygenist camp

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? Phillip Compeau & Pavel Pevzner 2017. All rights reserved.

would come to house a great number of racist and eugenicist supporters, since their theory allowed for a crystalline view of human race.

The issue of human origins is one more biological question that the advent of genetic data would help resolve. In 1987, Allan Wilson (along with Rebecca Cann and Mark Stoneking) three scientists provided resounding support for monogenism by demonstrating that all modern humans share a common female ancestor who lived approximately 200,000 years ago in Africa. Occam's razor led Wilson to propose what became known as the Out of Africa hypothesis: despite the earlier movements of Homo erectus, all non-Africans trace their roots to a recent migration of modern humans from Africa. Subsequent, more accurate, research has dated this migration as taking place approximately 70,000 years ago.

The Out of Africa hypothesis helps explain a substantial evolutionary riddle. The fossil record indicates that Neanderthals inhabited Europe from several hundred thousand years ago until approximately 40,000 years ago, at which point they were quickly replaced by the physically weaker Cro-Magnons, who resemble modern humans. Polygenists are forced to conclude, illogically, that Neanderthals evolved practically overnight into the very different Cro-Magnons. Yet the Out of Africa hypothesis permits the explanation that Neanderthals were a separate species, Homo neanderthalensis, who were less fit than the Cro-Magnons and were displaced over a period of hundreds of generations. This conclusion has been supported by fossil evidence, which shows that Neanderthals and Cro-Magnons coexisted in the same regions before Neanderthals' extinction.

We are left with several questions. First, how exactly does genetic evidence support the Out of Africa hypothesis? During their coexistence in Europe, was any romance kindled between Neanderthals and Cro-Magnons, and if so, how much? What were the paths that modern human populations took as they emigrated from Africa across the globe? And finally, how have these distinct populations contributed to your own genetic identity?

Mitochondrial DNA Confirms the Out of Africa Hypothesis

Mitochondria provide record of female inheritance

Billions of years ago, in the darkness of the primordial ocean, an ancient cell engulfed a bacterium. The bacterium was more efficient at producing energy than the cell, and so the two organisms became symbiotic; as the cell replicated, so did the bacterium within it. Over the eons, the bacterium slowly lost its identity as a distinct organism, and it eventually became an organelle serving as the energy center of the eukaryotic cell, which we now know as the mitochondrion (Figure 2).

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? Phillip Compeau & Pavel Pevzner 2017. All rights reserved.

Figure 2: Diagram of an animal mitochondrion. Courtesy Mariana Ruiz Villarreal.

Many researchers support this origin story for the mitochondrion in part because it has retained its own mitochondrial genome (mtDNA), a short circular chromosome that replicates independently of nuclear DNA. Most mtDNA comprises just 37 genes, all of which are critical to mitochondrial functions. Only 13 of these genes encode proteins ? the rest are translated into noncoding RNAs. Furthermore, mtDNA has practically uniform length across species --- throughout the animal kingdom, the mitochondrial genome is approximately 16,000 nucleotides long (in humans, the exact number of nucleotides is typically 16,569). The mitochondrion even has its own genetic code, as four of the 64 RNA codons translate into a different amino acid in the mitochondrion than they do in the nucleus.

The mitochondrial genome's short, uniform length across different species make it a perfect candidate for inexpensive comparative studies (Frederick Sanger sequenced human mtDNA all the way back in 1981). However, the mitochondrion has one additional interesting property: in mammals, sperm mitochondria are usually destroyed during fertilization. This means that you almost certainly inherited all your mitochondrial DNA from your mother, who inherited hers from her mother, and so on, back through the centuries to the most recent common female ancestor of all modern humans, who is commonly called mitochondrial Eve. A human phylogeny from mitochondrial DNA

But how old was Eve, exactly? Since mtDNA acquires mutations over time, we can construct an evolutionary tree from modern mitochondrial genomes. Furthermore, in Chapter 9, we saw that the rooted tree constructed by UPGMA presumes an "evolutionary clock", in which the age of an internal node corresponds to its distance from the leaves beneath it. If we use UPGMA to construct the desired tree, then the age of the root will correspond to the age of mitochondrial Eve.

Exercise Break: Apply UPGMA to construct an evolutionary tree based on human mtDNA data (click here for data). You can use MEGA to generate this tree.

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? Phillip Compeau & Pavel Pevzner 2017. All rights reserved.

As shown in Figure 3, which contains an evolutionary tree of human mtDNA, all nonAfricans clump together on one side of the root, but Africans can be found on both sides of the tree. In other words, Africans must be more genetically diverse than non-Africans, a result that has been confirmed by more advanced subsequent studies.

STOP and Think: What is the Occam's Razor explanation of the fact that Africans are more diverse than non-Africans?

If non-Africans clump together on one side of the tree of modern humans, then it goes to reason that they share a more recent common ancestor than all humans do. This deduction led Wilson to propose the out of Africa hypothesis, proposing that this nonAfrican common ancestor must have migrated out of Africa relatively recently. The argument is far from a proof, but it gives damning evidence against polygenism.

H1h1_Finnish H7j1_Ashkenazi HV4a1_Iranian V_Finnish V3b1_English B4d1_Chinese N9a10_Chinese Y2a1_Indonesian Y2a1_Malaysian L3e5a1_Morocco X2B_American X2d_Polish U6a7a1_English R2c_Saudi_Arabian F1a3_Filipino F3b1_Taiwanese R6_Thai A2f1a_Native_American A4-A200G_Chinese T1a1_Finnish T2a1a_Turkish T2b16_French K1a4a1_Serbian K2a3_Dutch B2_Argentinian W1c_Swedish W6_Bulgarian N1a1a3_Yemeni I1a1a3_Scottish I3_Irish I4a_Irish J1b_Armenian J2a2d1_Tunisian J2b1_Russian U2e2a4_Serbian L4b2a1_Yemeni C1d3_Uruguayan C4a1_Turkish M23_Yemeni M60_Indonesian D1_Argentinian D4e1a1_Chinese L2a1a_Mozambican L2e1_Sudanese L1c1d_Central_African

Figure 3: A rooted evolutionary tree constructed by UPGMA from a multiple alignment of complete mitochondrial genomes taken from present-day humans. There are two main subtrees derived from the ancestor. One contains exclusively Africans; the other contains a wide variety of individuals, including Africans.

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