| 2801/
Charles Darwin's On the Origin of Species was first published
on the 24th of November in 1859.
2802/ In the absence
of an adequate fossil record, for a long time it was essentially
impossible to determine the geological age of many evolutionary
lineages. However, Zuckerkandl and Pauling (1962) showed that
many, perhaps most, molecules have a rather constant rate
of change over time. Such molecules can serve as a molecular
clock. Well-dated fossils with modern descendants
provide us with a yardstick for calibrating a given molecular
clock.
2803/ It was by the molecular
clock method that the branching point between chimpanzee and
man was shown to be as recent as 5-8 million years ago, rather
than 14-16 million years, as had been previously generally
accepted.
2804/ The earliest fossil
life was found in strata about 3.5 billion years old. These
earliest fossils are bacteria like, indeed they are remarkably
similar to some blue-green bacteria and other bacteria that
are still living.
2805/ About a third of
the early fossil species of prokaryotes are indistinguishable
from still living species and nearly all of them can be placed
in modern genera.
2806/ After about 1,000
million years of exclusively bacterial life on Earth, perhaps
the most important and dramatic event in the history of life
took place - the origin of the eukaryotes. Eukaryotes differ
strikingly from prokaryotes by the possession of a nucleus
surrounded by a membrane and containing individual chromosomes.
2807/ Genome size is
measured in terms of the number of base pairs, although for
practical reasons the units are megabases (1,000 base pairs,
abbreviated Mb). The genome of humans is about 3500 Mb. In
a bacterium it may only be about 4 Mb. Very large figures
are also found in salamanders and lungfishes.
2808/ When the pharaohs'
tombs were opened in Egypt early in the nineteenth century,
not only human mummies were found but also those of sacred
animals such as cats and ibises. When the anatomy of these
animal mummies, estimated to be about 4000 years old, was
carefully compared by zoologists with living representatives
of these species, no visible differences could be found.
2809/ Certain deep-sea
fish have dwarf males that are attached to the females, because
free-swimming males might have difficulty finding females
in these vast and rather lifeless spaces.
2810/ In certain species
of seals, like the elephant seal, males may be several times
larger than females because larger males can better defeat
their rivals in territorial fights and so acquire larger harems.
2811/ Emperor Penguins
court and lay their single egg under the most adverse conditions
at the beginning or in the middle of the Antarctic winter,
a season of frequent blizzards. The advantage of this timing
is that the young hatch at the beginning of the Southern Spring
and are raised during the southern summer, when conditions
for their survival and growth are at an optimum.
2812/ Darwin marvelled
that such a wonderful structure as an eye could have evolved
through natural selection, but comparative anatomists have
shown not only that eyes evolved in the animal species at
least 40 times independently, but also that among the existing
photosensitive organs every intermediate step is found between
a simple light-sensitive spot on the epidermis and a perfect
eye with all its accessories.
2813/ About three new
species of bird are discovered each year.
2814/ In 1758 Linnaeus
knew some 9,000 species of plants and animals. By now about
1.8 million species of animals have been described (excluding
agamospecies) and the grand total of species is estimated
to be at least 5 to 10 million. Most of these live in the
canopy of the tropical rain forest and, with 1-2 percentof
this forest being destroyed every year, this number will soon
be reduced appreciably.
2815/ There are currently
thought to be about 9,800 species of warm-blooded aerial birds
and 4,800 species of terrestrial warm blooded mammals.
2816/ Beetles are the
largest group of animals. It is believed that the number of
species of beetles range from approximately 250,000 to 350,000.
2817/ Almost 50 percent
of North American species of cricket were discovered only
by their different songs, they are that similar to each other.
2818/ During a drought
period in the Pliocene (ca. 6 million years ago), the softer
grasses in North America were largely replaced by harsher
grasses, which had three times as much silica content.
2819/ Among the browsing
horses, all species became extinct except those with the longest
teeth.
2820/ Particularly in
the Americas, the native populations were ravaged by epidemics
caused by European infectious diseases, particularly smallpox.
The native population of the Americas, which was estimated
to have been 60 million when Columbus first landed in the
Bahamas, had crashed to 5 million only 20 years later.
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