Encyclopedia of Evolution.pdf - Online Reading Center

apes
that there are many (perhaps an infinite number of) universes,
each one with different cosmological properties.
According to this multiverse model, most of these universes
have characteristics in which the evolution of intelligence
was impossible (they expand too fast, or they form into
clouds of helium, or they collapse into black holes). But in
one of them—the one humans know—the conditions were
just right. In an infinite number of universes, Goldilocks has
to exist somewhere. Since there can be no evidence of other
universes, this possibility remains forever outside the realm
of scientific investigation.
Further Reading
Davies, Paul. Cosmic Jackpot: Why Our Universe Is Just Right for
Life. New York: Houghton Mifflin, 2007.
apes See primates.
archaebacteria Archaebacteria (now called archaea by
scientists) are single-celled organisms that resemble bacteria.
Most of them live in conditions of extreme acidity,
temperature, salinity, or pressure that biologists at one time
considered impossible for organisms to endure. Pyrolobus
fumarii, for example, lives in deep ocean vents where the
water reaches 235°F (113°C). Water boils at 212°F (100°C)
at the air pressure of sea level, but the pressure at the bottom
of the ocean allows superheated water to exist, in which this
archaebacterium lives.
Like eubacteria (see bacteria, evolution of), archaebacteria
are prokaryotic, which means that their DNA consists
of circular strands that float freely in the cell fluid rather
than being enclosed in a nucleus (see DNA [raw material
of evolution]). In contrast, eukaryotic cells have DNA in
the form of linear chromosomes, inside of a membrane-bound
nucleus (see eukaryotes, evolution of).
Because very small organisms generally come in just a
few fundamental shapes, archaebacteria look like eubacteria
and for decades were classified with them. Archaebacteria
have some characteristics that distinguish them from eubacteria,
including the following:
• All cells have membranes that are built of phospholipidtype
molecules. In eubacteria and eukaryotic cells, the
phospholipids are built from unbranched fatty acids linked
to glycerol. In archaebacteria, the phospholipids are built
from branched isoprenoids linked differently to a chemically
backward form of glycerol. This is one of the most fundamental
differences that are known to occur among cells.
• Archaebacteria have, in addition to cell membranes, cell
walls that are chemically different from those of eubacteria.
• The transfer RNA and ribosomes of archaebacteria differ
from those of eubacteria.
• The DNA of archaebacteria is associated with histone proteins,
which is a characteristic that they share with eukaryotic
cells, but not with eubacteria.
Archaebacteria resemble what most evolutionary scientists
consider to be the most primitive life-forms on Earth.
Not only are they structurally simple, but the extreme condi-
tions in which they live resemble those of the earliest oceans.
During the earliest Precambrian time, the atmosphere and
oceans contained no oxygen gas. All modern archaebacteria
are obligate anaerobes, which means that they cannot live
in the presence of oxygen gas. Many of the eubacteria that
are identified by DNA analysis as most primitive also live in
extreme conditions.
Carl Woese (see Woese, Carl R.) began making nucleic
acid comparisons among many different species in the 1970s
(see DNA [evidence for evolution]). At that time, all bacteria
were lumped together into one category. Woese found
that certain bacteria that lived in extreme conditions had
nucleic acid sequences as different from those of other bacteria
as they were from those of eukaryotes. It was from this
discovery that evolutionary scientists began distinguishing the
archaebacteria as a distinct branch of life from the eukaryotes
and the eubacteria (see tree of life). Scientists prefer
the term Archaea, because archaebacteria are as genetically
different from the more familiar bacteria as they are from
eukaryotes.
DNA studies since the time of Woese’s original work
have continued to confirm the uniqueness of archaebacteria.
The complete genome of Methanococcus janaschii
was published in 1996. Only 11–17 percent of its genome
matched that of known eubacteria, and over half of its
genes are unknown in either bacteria or eukaryotes. Moreover,
some of the DNA similarity between archaebacteria
and eubacteria may be due to horizontal gene transfer,
in which bacteria exchange small segments of DNA.
Although most horizontal gene transfer occurs between
bacteria closely related to one another, exchange between
archaebacteria and eubacteria has occurred frequently during
the history of life.
Three major groups of archaebacteria are recognized by
many evolutionary scientists on the basis of DNA sequences:
• Euryarcheota include the methanogens and the halophiles.
Methanogens convert hydrogen gas (H 2) and carbon
dioxide gas (CO 2) into methane gas (CH 4). They live
in marshes and in the intestinal tracts of animals such as
cows and humans. They are a principal source of methane
in the atmosphere (see Gaia hypothesis). Halophiles live
in extremely salty conditions such as the Dead Sea and the
Great Salt Lake. Halophiles may have evolved more recently
than other archaebacteria. First, they obtain their carbon
from organic molecules produced by the decay of other
organisms, which implies that they evolved after these
other organisms were already in existence. Second, they
use a molecule that resembles the rhodopsin visual pigment
in the vertebrate eye in order to produce energy from sunlight.
Third, DNA analyses place halophiles out toward the
branches of the archaebacterial lineage, rather than near
the primitive base of the lineage.
• Crenarcheota include thermophiles that live in very hot
environments, but some crenarcheotes live in soil and water
at moderate temperatures.
• Korarcheota have been identified only by their DNA
sequences and little is yet known about them.