Page 212 - Introduction to Paleobiology and The Fossil Record

THE ORIGIN OF LIFE 199

Nucleus
(n) n
n

n

New cell wall
(b)
(a)

Spore tetrads Septum Branched tubular
filament

(d)

(c)
Figure 8.11 Early fossil “eukaryotes”. (a) The thread-like Grypania meeki, preserved as a
carbonaceous film, from the Greyson Shale, Montana (c. 1.3 Ga). (b, c) Single-celled eukaryotes from

the Bitter Springs Chert, Australia (c. 800 Ma): (b) Glenobotrydion showing possible mitosis (cell
division in growth), and (c) Eotetrahedrion, probably a cluster of individual Chroococcus-like
cyanobacteria. (d) Branching siphonalean-like filament. Scale bars: 2 mm (a), 10 μm (b–d). (Courtesy of

Martin Brasier, based on various sources.)

branched filaments that look like modern

siphonalean green algae (Fig. 8.11d).
Older fossils too look like algae. For
example, in the Lakhanda Group of eastern
Siberia, 1000–950 Ma, five or six metaphyte

species have been found (Fig. 8.12), as well as
a colonial form that forms networks rather
like a slime mold. But the key fossil in under-
standing early eukaryote evolution is Bangio-
morpha (Box 8.3).

Multicellularity and sex

As eukaryotes ourselves, multicellularity and
sex seem obvious. Prokaryotes are single-
celled organisms, although some form fi la-
ments and loose “colonial” aggregations.
True multicellular organisms arose only
among the eukaryotes. These are plants and
animals that are composed of more than one
cell, typically a long string of connected cells
in early forms. Multicellularity had several
important consequences, one of which was

that it allowed plants and animals to become Figure 8.12 A filamentous alga from the
large (some giant seaweeds or kelp, forms of Lakhanda Group, Siberia (c. 1000 Ma), 400 μm
algae, reach lengths of tens of meters). Another wide. (Courtesy of Andy Knoll.)
consequence of multicellularity was that cells