Page 197 - Introduction to Paleobiology and The Fossil Record

184 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
Origins are among the deepest questions:
Where did humans come from? Where did life
itself come from? The most ancient philoso-
phers could see that the world is made of
living and non-living things, and they wanted
to know where the spark of life came from.
How do you go from a non-living thing, like
a rock or a glass of water, to a living thing,
like a plant or an animal?
These early speculations led to many cre-
ation myths, stories about how the non-living
to living transition might have taken place.
Creation myths are common to many reli-
gions, and they explain the origin of life by
divine intervention. These ideas are not scien- laid their eggs on rotting meat, the eggs
tific, however, because they cannot be tested.
We explored the issue of creationism in
Chapter 5.
The current scientific view is that life arose
on the Earth some time before 3.5 Ga (Ga =
giga years old, or 1000 Ma). In rocks from
Australia and South Africa dated at around
3.5 Ga, isotopes of carbon are consistent with
the presence of a marine biosphere that pref-
12
erentially incorporated the carbon-12 (C )
13
isotope into organic matter relative to C .
The first organisms were simple, single-celled
prokaryotes similar to modern microbes.
More complex cells, eukaryotes, arose only
later, perhaps 2.7 Ga, and much later than
that came the first true plants and animals.
This means that the fi rst three-quarters of
the history of life passed by in the company
of organisms that were neither plant nor
animal.
In this chapter, we look fi rst at different
ways of explaining origins. Then, we go on to
look at the diversity of evidence about when
and how life arose. We concentrate on the
geological and fossil evidence, of course, but
include some necessary molecular biology and
biochemistry as well.
THE ORIGIN OF LIFE
Scientifi c models
There have been many scientific models for
the origin of life, some of them now rejected
by the evidence, and others still available as
potentially valid hypotheses:
1 Spontaneous generation.
2 Inorganic model.
3 Extraterrestrial origins.
4 Biochemical model.
5 Hydrothermal model.
Medieval scholars believed that many
organisms sprang into life directly from non-
living matter, a form of spontaneous genera-
tion. For example, frogs were said to arise
from the spring dew and maggots were said
to come to life in rotting fl esh. However,
careful tests proved that there was no truth in
these ideas. Louis Pasteur in 1861 enclosed
pieces of meat in airtight containers, and
maggots did not appear. He showed that fl ies
hatched as maggots and the maggots then
turned into flies. So, the idea of the origin of
life by spontaneous generation is a scientifi c
hypothesis because it may be tested, but it
turns out to have been wrong. It is import-
ant to realize that scientific and non-scientifi c
do not mean “right” and “wrong”: science is
about testing and rejecting alternate hypoth-
eses until one remains that is not rejected.
The inorganic model for the origin of life
is that complex organic molecules arose grad-
ually on a pre-existing, non-organic replica-
tion platform – silicate crystals in solution.
Silicate crystals, clay minerals, were subject to
selection pressures on the ancient seabed, and
then organic molecules became involved and
the inorganic selection became organic. This
view has been championed vigorously by
Graham Cairns-Smith of Glasgow University,
but it has not gained widespread support. The
first experiments to test the model were carried
out in 2007, but they were not conclusive.
The extraterrestrial model is that the build-
ing blocks for life were seeded on Earth from
outer space. Simple molecules, such as hydro-
gen cyanide, formic acid, aldehydes and acet-
ylenes are found in certain classes of meteorites
called carbonaceous chondrites, as well as in
comets, and these chemicals might have been
delivered to the surface of the Earth during a
phase of massive meteorite bombardment
about 3.8 Ga. In other, more extreme, forms
of this hypothesis, DNA might even exist in
space, or life in its entirety might have evolved
elsewhere in the universe, and was seeded on
the Earth during the Precambrian.
Collectively, these views have sometimes
been called “panspermia”, meaning “univer-
sal seeding”. The panspermia model received