Evolution, 4th Edition

556 CHAPTER 21

of humans out of Africa. Most of the branches connecting non-Africans are rela-

tively short, consistent with humans’ rapid and relatively recent colonization of the

planet. Further genetic signatures of the expansion of humans across Earth are

seen in nuclear genes: the highest nucleotide diversity and the lowest linkage dis-

equilibrium in humans are found in populations in southern Africa [26], and het-

erozygosity declines the farther a population is from Africa (see F i g u r e 7.7). T hat

is just the pattern we expect, since genetic bottlenecks resulted as small groups of

intrepid ancient explorers colonized new regions.

The human history of hybridization

As humans spread out across the Middle East, Europe, and Asia, they encountered

the Neanderthals and Denisovans, whose ancestors had left Africa more than

500 Ky earlier. Although we don’t know much about how humans interacted with

those other two groups, one thing is clear: they hybridized, and did so more than

once [50, 52, 77]. Pulses of hybridization happened at different times and in differ-

ent places. These liaisons left modern humans a checkered genetic legacy (FIGURE

21.14). Since the matings occurred outside Africa, living Africans have little or no

DNA from Neanderthals or Denisovans. In contrast, about 2 percent of the DNA

in modern Europeans and Asians comes from Neanderthals, as a result of at least

two bouts of hybridization. Living Melanesians have DNA inherited from both

Neanderthals and Denisovans.

Neanderthals contributed advantageous alleles to the human gene pool that

affect skin and immune traits [76, 77]. In Chapter 7 you saw that an allele in the

gene EPA S1 helped Tibetans adapt to life at high elevations (see F i g u r e 7. 2 3). T hat

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_21.13.ai Date 01-18-2017

(A) (B)

Bonobos

Chimpanzees

Living

humans

Neanderthals

Australian

Pima (No. American)

Papua New Guinea highland

Georgian

Uzbek (central Asian)

Crimean Tatar

French

English

Korean

Chinese

Asian Indian

Khirgiz

Warao

Siberian Inuit

Guarani (So. American)

Japanese

Mkamba

Ewondo

Lisongo

Yoruba

Mandenka

Efk

Ibo

Mbenzele

Mbenzele

Biaka

Kikuyu

Hausa

Mbuti

San

Non-African

African

(central Asian)

(So. American)

FIGURE 21.13 Mitochondrial gene

trees reveal relations among humans

and closely related species. (A) A gene

tree based on the mtDNA from living

humans, Neanderthal fossils, chimpan-

zees, and bonobos. The similarity of se-

quences from Neanderthals and living

humans is a dramatic confirmation of

the conclusion made earlier from fossils

that the two lineages are very closely

related. In this tree, the lengths of

branches are proportional to the num-

ber of changes in the DNA sequences.

(B) A gene tree of mtDNA from living

humans shows that its deepest branch-

es are found in Africa, corresponding to

where our species originated. All non-

African lineages are descendants of a

single ancestral mtDNA that left Africa

about 60 Kya. The shallower branches

connecting all non-African lineages

indicate that they are more recently

diverged than lineages in Africa. (A after

[5]; B after [33].)

21_EVOL4E_CH21.indd 556 3/22/17 1:51 PM