Biology Reference
In-Depth Information
and the morphology of
73,060
eukaryotic species by a parsimony method. Their
results indicated a high degree of congruence with the major taxonomic groups,
with only a small number of misplaced species. They concluded that tree-
calculation algorithms can retrieve phylogenetic trees for very large data sets.
12.7 The Universal Tree of Life
As the following sections show, the evolution of life, particularly early life,
remains controversial and difficult to study. Because arthropods evolved early
and diverged rapidly, it has been especially difficult to resolve their early evolu-
tionary history (
Trautwein et al. 2012
).
12.7.1 Two Domains
The traditional view was that life is divided into animals and plants, and the
study of bacteria and fungi often took place in departments of botany. Later,
it was realized that organisms could be divided into prokaryotes or eukaryotes
(organisms without or with nuclei bounded by nuclear membranes and contain-
ing mitochondria). It became clear, however, that fungi are not plants (molecular
data indicate they are actually more closely related to animals). All single-celled
eukaryotes initially were placed into the phylum Protista, but this was found to
be a heterogeneous group consisting of algae (formerly plants), protozoa (for-
merly animals), water molds (formerly fungi), and others (
Mayr 1998
). The group-
ing of living organisms was modified as more information became available.
12.7.2 Three Domains
A more recent view is that there are three primary domains of life
(Archaebacteria, Eubacteria, and Eukaryota) (
Woese et al. 1990, Doolittle 1999,
Woese 2000
). The “Bacteria” were separated into two groups on the basis of
variation in the small subunit rRNA: the traditional bacteria (called Eubacteria)
and a previously unrecognized group (Archaebacteria or Archaea) that contain
members that inhabit extreme environments such as hot springs, sulfur springs,
and deep vents (
Woese 1987
).
Woese (1987)
considered the Archaea might
have been the first organisms on earth because they inhabit such extreme envi-
ronments, although now we know Archaea are present in less-extreme habi-
tats (
Brochier-Armanet et al. 2011
). However, separation of “bacteria” into two
domains of a rank equal to that of the Eukaryota was justified because they were
assumed to have evolved independently from a precursor group (
Figure 12.6
).
The Archaebacteria (sometimes called Archaea) were considered to be as differ-
ent from the Eubacteria as the Eubacteria were from the Eukaryota on a molecu-
lar basis.
