Geology Reference
In-Depth Information
Cells larger than 60 microns appear in abundance at least
1.4 billion years ago, and many show an increased degree of
organizational complexity compared to that of prokaryotic
cells. An internal membrane-bounded cell nucleus is present
in some, for instance. Furthermore, microscopic, hollow fossils
known as
acritarchs
that probably represent cysts of planktonic
algae become common during the Meso- and Neoproterozoic
(
Figure 19.23a, b). And vase-shaped microfossils from rocks
in the Grand Canyon have also been tentatively identifi ed as
cysts of some kind of algae (Figure 19.23c).
◗
Endosymbiosis and the Origin of Eukaryotic Cells
Accord-
ing to a widely accepted theory, eukaryotic cells formed
from several prokaryotic cells that entered into a symbiotic
relationship. Symbiosis, involving a prolonged association
of two or more dissimilar organisms, is common today. In
many cases, both symbionts benefi t from the association, as
in lichens, which were once thought to be plants, but actually
are symbiotic associations between fungi and algae.
In a symbiotic relationship, each symbiont must be
capable of metabolism and reproduction, but the degree of
dependence in some relationships is such that one symbiont
cannot live independently. This may have been the case with
Proterozoic symbiotic prokaryotes that became increasingly
interdependent until the unit could exist only as a whole. In
this relationship, though, one symbiont
lived within the other, which is a special
type of symbiosis called
endosymbiosis
(
0
10
µ
m
◗
Figure 19.20
Archean Fossil Photomicrograph and schematic
restoration of fossil prokaryote from the 3.3- to 3.5-billion-year-old
Warrawoona Group, Western Australia.
the 2.1-billion-year-old Negaunee Iron Formation of Michi-
gan has fossils known as
Grypania
, which is the oldest known
megafossil, but it was probably a single-celled bacterium or
some kind of algae (
◗
Figure 19.22).
Figure 19.24).
Supporting evidence for endo-
symbiosis comes from studies of liv-
ing eukaryotic cells containing internal
structures called organelles, such as mi-
tochondria and plastids, that have their
own genetic material. In addition, pro-
karyotic cells synthesize proteins as a sin-
gle system, whereas eukaryotic cells are
a combination of protein-synthesizing
systems. That is, some of the organelles
within eukaryotic cells are capable of
protein synthesis. These organelles with
their own genetic material and protein-
synthesizing capabilities are thought
to have been free-living bacteria that
entered into a symbiotic relationship,
eventually giving rise to eukaryotic cells.
◗
Monera
Bacteria
Archaea
DNA
Ribosomes
Cytoplasm
Plasma membrane
Cyanobacteria
Pili
Cell
wall
Bacterial
flagellum
Capsule
Prokaryotic cell
⎪
⎭
Nuclear envelope
Nucleoplasm + DNA
Nucleolus
Nucleus
Microtubules
Protista
Protozoans, algae
Vesicle
Lysosome
Microfilaments
Fungi
Spores
Plantae
Plant tissue cells
Rough
endoplasmic
reticulum
Smooth
endoplasmic
reticulum
Golgi body
The Dawn of Multicelled Organ-
isms
Multicelled organisms
are not
only composed of many cells, but also
have cells specialized to perform specifi c
functions such as reproduction and
respiration. Unfortunately, the fossil
record does not tell us how multicelled
organisms arose from single-celled an-
cestors. However, studies of present-day
Animalia
Animal tissue cells
Plasma
membrane
Mitochondrion
Pair of centrioles
Vesicle
Eukaryotic cell
◗
Figure 19.21
Prokaryotic and Eukaryotic Cells Eukaryotic cells have a cell nucleus
containing the genetic material and organelles such as mitochondria and plastids. In
contrast, prokaryotic cells are smaller and not nearly as complex as eukaryotic cells.
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