Essential Cell Biology 5th edition

Examining the Human Genome
327
region inhabited
by Neanderthals
45K
25K
41K
45K
40K
1.5K
~200K
interbreeding
of humans and
Neanderthals
(~55K years ago)
0.8K
0.8K
15K
Figure 9–37 Ancestral humans encountered Neanderthals on their way out of Africa. Modern humans descended
from a relatively small population—perhaps as few as 10,000 individuals—that existed in Africa approximately 200,000 (200
K) years ago. Among that small group of ancestors, some migrated northward, and their descendants spread across the
globe. As ancestral humans left Africa, around 130,000 years ago (purple arrows), they encountered Neanderthals who
inhabited the region indicated in light blue. As a result of interbreeding (in the region shown in dark blue), the humans that
subsequently spread throughout Europe and Asia (red arrows) carried with them traces of Neanderthal DNA. Ultimately,
ancestral humans continued their global spread to the New World, reaching North America approximately 25,000 years
ago and the southern regions of South America 15,000 years later. This scenario is based on many types of data, including
fossil records, anthropological studies, and the genome sequences of Neanderthals and of humans from around the world.
(Adapted from M.A. Jobling et al., Human ECB5 Evolutionary n9.101/9.35 Genetics, 2nd ed. New York: Garland Science, 2014.)
Genome Variation Contributes to Our Individuality—But
How?
With the possible exception of some identical twins, no two people have
exactly the same genome sequence. When the same region of the genome
from two different humans is compared, the nucleotide sequences typically
differ by about 0.1%. This degree of variation represents about 1
difference in every 1000 nucleotide pairs—or some 3 million genetic
differences between the genome of one person and the next. Detailed
analyses of human genetic variation suggest that the bulk of this variation
was already present early in our evolution, perhaps 200,000 years
ago, when the human population was still small. Yet much of this variation
has been reshuffled as more and more generations of humans have
arisen. Thus, although a great deal of the genetic diversity in present-day
humans was inherited from our early human ancestors, each individual
inherits a unique combination of this ancient genetic variation.
Sprinkled on top of this “tossed salad” of ancient variation are mutations
that are much more recent. At birth, each human’s genome contains
approximately 70 new mutations that were not present in the genomes of
either parent. Combined with the jumbled collection of ancient variation
we acquired from our ancestors, these recent mutations further distinguish
one individual from another. Most of the variation in the human
genome takes the form of single base-pair changes. Although some of
these base-pair changes are unique to individual humans, many more
are preserved from our distant ancestors and are therefore widespread in
the human population. Those single-base changes that are present in at
least 1% of the population are called single-nucleotide polymorphisms
(SNPs, pronounced “snips”). These polymorphisms are simply points in
the genome that differ in nucleotide sequence between one portion of
the population and another—positions where, for example, more than 1%