Human Evolution

Human beings belong to the family of Hominidae.
Within this family they are a single
living species, Homo sapiens. Studies of DNA
sequences obtained from different parts of the
human genome from populations living in
different regions of the world indicate that the
genetic diversity of humans living today is surprisingly
limited. The origin of today’s humans
can be traced back to about 100000 to 300000
years ago in Africa. From here, an anatomically
modern H. sapiens ancestor spread out over the
earth and diversified. About 90% of the overall
genetic variation in humans is among individuals
and only 10% among different ethnic groups.
There are no races of H. sapiens; rather, different
ethnic groups have developed in different
geographic regions during the past 30000–
40000 years

Hominid family tree with uncertain relationships

The oldest identified hominid skeletal remains
have been found in Eastern Africa. They are attributed
to an extinct genus, Australopithecus.
Several different species originated about 4.5 to
2 million years ago during the Pliocene epoch
(5.3 to 1.6 million years ago). During this time
fundamental changes in morphology and behavior
occurred, presumably to adapt to changing
the habitat from the forest to the plains:
bipedalism, a dramatic increase of brain
volume, and anatomic changes in the pharynx
to allow speech were accompanied by tool
making and other complex behavior.
The earliest fossils of Homo sapiens are from
100000 years ago. Modern humans as they
exist today date back about 30000–40000
years.

Geographic distribution of important hominid find

Two hypotheses for the origin of modern
humans have been put forward. The first is that
evolution was multiregional, that is, early
humans migrated to different geographic regions,
resulting in parallel evolution throughout
the world. The other hypothesis assumes
that all modern humans moved through the
middle east into Europe and Asia between
50000 and 100000 years ago and replaced

Relationship of modern humans and Neandertals

Modern humans and Neandertals coexisted
about 30000–40000 years ago. Recent studies
indicate that Neandertals did not contribute
mitochondrial DNA to modern humans. At two
different locations about 2000km apart,
mtDNA from Neandertal specimens at Feldhofer
Cave, Neandertal, and Mezmaiskaya Cave
in the northern Caucasus showed only 3.48%
sequence divergence.

Phylogenetic tree reconstruction from mitochondrial DNA

Genetic studies are consistent with the Out of
Africa hypothesis. The strongest evidence came
froman analysis of mitochondrial DNA from147
modern humans of African, Asian, Australian,
New Guinean, and European origin. The
genealogical tree could be traced back to an ancestral
haplotype presumably 200000 years old
(“mitochondrial Eve”).

Genome Analysis by DNA Microarrays

A microarray or DNA chip is an assembly of
oligonucleotides or other DNA probes, e.g.,
cDNA clones, fixed on a fine grid of surfaces. It is
used to analyze the expression states of a series
of genes represented in cDNA prepared from
mRNA (expression screening) or to recognize
sequence variations in genes (screening for
DNA variation). The advantages of using microarrays
are manyfold: simultaneous large-scale
analysis of thousands of genes at a time, automation,
small sample size, and easy handling
given the right equipment. Several manufacturers
offer highly efficient microarrays that
can accomodate 300000 DNA probes on a highdensity
glass slide of small size (e.g.
1.28cm!1.28 cm).
Two basic types of DNA microarrays can be distinguished,
although many variations are being
developed: (i) microarrays of previously prepared
DNA clones or PCR products that are attached
to the surface, arranged in a high-density
gridded array in two-dimensional linear
coordinates; (ii) microarrays of oligonucleotides
synthesized in situ on a suitable surface.
Both types of DNA arrays can be hybridized
to labeled DNA probes in solution.

Gene expression profile by cDNA array

This figure shows amicroarray of 1500 different
cDNAs from the human X chromosome. The
cDNAs were obtained from lymphoblastoid
cells of a normal male (XY) and a normal female
(XX). The cDNAs of the male cells were labeled
with the fluorochrome Cy3 (green) and the
cDNAs of the female cellswere labeledwith Cy5
(red). The inactivation of most genes in one of
the two X chromosomes in female cells leads to
a 1:1 ratio of cDNAs from the expressed genes
in the male and the female X chromosomes
(yellowsignal at most sites because red fluorescence
(female) and green fluorescence (male)
signals are superimposed owing to the similar
expression levels in male and female cells). An
exception is the XIST gene, which regulates X inactivation
(see p. 228). It is expressed on the inactive
X chromosome only.