Page 201 - Introduction to Paleobiology and The Fossil Record

188 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
waves in the water. The experiments are
complex, and investigators are continuing to
explore the behavior of simple RNA replicase,
self-replicating vesicles and how the two could
come to function together (Szostak et al.
2001).
If the RNA world existed, when was this
and for how long? The Earth had to be cool
enough for the organic elements to survive
being burned off, and the RNA world must
pre-date any traces of modern forms of life.
Some estimate that this might have been a
time of 100–400 myr, somewhere between
4.0 and 3.5 Ga.
EVIDENCE FOR THE ORIGIN OF LIFE
The Early Precambrian world
The Precambrian is divided into the Hadean,
Archaean and Proterozoic eons. The Hadean
Eon spans from the origin of the Earth, 4.57
to about 4 Ga (Fig. 8.3). At fi rst, the Earth
Ma
0
humans
Phan. dinosaurs MAMMALS
abundant life VERTEBRATES
Ediacara METAZOANS
1000 Bitter Springs Chert MULTICELLS now happened mainly round the margins of
Proterozoic EUKARYOTES
2000 oxygenation
Gunflint Chert
of atmosphere
Precambrian
3000 Fig Tree Chert
Archaean
Apex Chert, Australia
origin of life PROKARYOTES
oldest rocks
4000
Hadean
origin of the Earth
Figure 8.3 Time scale showing major events in
the history of the Earth and of life. Most of the
time scale is occupied by the Precambrian,
whereas the well-known fossil record of the
Phanerozoic (Phan.) accounts for only one-
seventh of the history of life.
was a molten mass, but it cooled, separating
into an outer cool crust and an inner molten
mantle and core. Massive volcanic eruptions
produced great volumes of gases: carbon
dioxide, nitrogen, water vapor and hydrogen
sulfide. At the very beginning of the Hadean,
temperatures on the Earth’s surface were too
high, and the crust was too unstable for any
form of carbon-based life to exist. During the
Hadean, the cratering record on the moon
suggests that there were a few ocean-vapor-
izing impacts on Earth – impacts from large
comets or asteroids that would have provided
enough energy to turn the ocean into steam.
Thus, if life had got started in the early
Hadean, it would have been wiped out, only
to start afresh. Also, smaller impacts at the
end of the Hadean would have destroyed life
on the surface of the Earth; only microbes
that could stand high temperatures living in
the subsurface would have survived.
As the Earth’s surface cooled, the litho-
sphere, its rocky crust, began to differentiate
as a cooler upper layer above the underlying
asthenosphere. As the rocky lithosphere
formed, magma convection became restricted
to the asthenosphere, and the upper crust
formed plates that were moved by mantle
convection. This marks the beginning of plate
tectonics (see p. 42). Heat loss from the Earth
ated with hydrothermal activity, began to
these early plates, and black smokers, associ-
form.
The oldest rocks are from Canada and are
dated at 3.8–4 Ga, and some mineral grains
from Australia have even been dated to
4.4 Ga.
The Archaean Eon lasted from about 4 to
2.5 Ga. The oldest sedimentary rocks have
been reported from the Isua Group in Green-
land, dated at 3.8–3.7 Ga. The rocks are hard
to interpret because they have been metamor-
phosed by heat and physical forces, but most
geologists accept that some of the Isua Group
rocks were originally sediments. Sedimentary
rocks prove that the crust had cooled and
rivers were flowing and eroding rocks.
The Isua Group rocks have also produced
controversial signatures of early life. Nobody
would expect to find fossils in these rocks
because they have been too metamorphosed,
but Rosing and Frei (2004) have reported evi-
dence that photosynthesis was happening