Gene Cloning and DNA Analysis: An Introduction, Sixth Edition ...

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Part III The Applications of Gene Cloning and DNA Analysis in Biotechnology
Next, the Neanderthal sequence was aligned with the equivalent sequences from 994
modern humans. The differences were striking. The Neanderthal sequence differed
from the modern sequences at an average of 27.2 ± 2.2 nucleotide positions, whereas
the modern sequences, which came from all over the world, not just Europe, differed
from each other at only 8.0 ± 3.1 positions. This degree of difference is incompatible
with the notion that modern Europeans are descended from Neanderthals. Similar
results were obtained when mitochondrial DNA from a second Neanderthal skeleton
was examined. The results therefore provide an independent proof of the Out of Africa
hypothesis, and show that, at least for Europe, the multiregional model is incorrect.
16.4.2 DNA can also be used to study prehistoric human
migrations
The modern humans that displaced Neanderthals arrived in Europe about 40,000 years
ago. This is clear from the fossil and archaeological records. But were these humans
themselves displaced by newer populations who migrated into Europe more recently?
The spread of agriculture into Europe
Some archaeologists have suggested that new populations of humans might have moved
into Europe during the past 10,000 years, and that these humans brought agriculture
into the continent. The transition from hunting and gathering to farming occurred in
southwest Asia about 10,000 years ago, when early Neolithic villagers began to cultivate
crops such as wheat and barley. After becoming established, farming spread into
Asia, Europe, and North Africa. By searching archaeological sites for the remains
of cultivated plants or for implements used in farming, two routes by which agriculture
spread into Europe have been traced. One of these trajectories followed the
Mediterranean coast to Spain and eventually to Britain, and the second passed along the
Danube and Rhine valleys to northern Europe (Figure 16.10).
One explanation for the spread of agriculture is that farmers moved from one place
to another, taking with them their implements, animals, and crops, and displacing the
indigenous, pre-agricultural communities that were present in Europe at that time.
This wave of advance model was initially supported by archaeogeneticists, as it agrees
with the results of a large phylogenetic study, carried out in the 1990s, of the allele
frequencies for 95 nuclear genes in populations from across Europe. This dataset was
analyzed by a technique often used in population genetics, called principal component
analysis, which attempts to identify patterns in the geographical distribution of alleles,
these patterns possibly being indicative of past population migrations.
The most striking pattern within the European dataset, accounting for about 28% of
the total genetic variation, is a gradation of allele frequencies running from southeast
to northwest across the continent (Figure 16.11). This pattern suggests that a migration
of people occurred either from southeast to northwest Europe, or in the opposite direction.
Because the former migration coincides with the spread of farming, as revealed by
the archaeological record, this first principal component was looked on as providing
strong support for the wave of advance model.
Using mitochondrial DNA to study past human migrations into Europe
Principal component analysis has one weakness when applied to past human migrations.
It is difficult to determine when a migration identified in this way took place.
This means that the link between the first principal component and the spread of