Evidence for Earliest Earth Life
Most of the evidence of the earliest life on Earth is in the form of
chemical tracers: there are few truly ancient fossils. This absence of a fossil record implies that
simple, unicellular life dominated the early Earth, a fact consistent with what
we know of unicellular life on the planet today, and of conditions necessary
for fossil preservation. In fact, until
about 2 Ga (billion years ago) there was little oxygen on the planet, and the
development of complex eukaryotic cells, which live via oxygenic respiration,
was probably not possible. Based on the study of ancient soils,it is believed that oxygen first appeared and rose rapidly in the atmosphere
approximately 2 Ga (Figure 2) and that it was only upon this rise that the
development of eukaryotic cells was possible.
The Cambrian explosion of species and complex multicellular eukaryotes
did not occur until approximately 500 million years ago, when oxygen reached
current levels. From that point onward,
the Earth began to take on what we would find a familiar appearance: occupied
by plants, animals, and fungi.
Figure 2. Evolution
of life on Earth as related to the appearance of oxygen in the atmosphere. The plot of oxygen versus time is modeled
after the data of Rye and Holland (1999), who have proposed this pattern as the
most likely based on studies of ancient soils (paleosols). The pictures of organisms are meant to
emphasize that the early Earth was colonized by simple organisms, probably
prokaryotic in nature, and that complex organisms (multicellular large
creatures) did not appear until the oxygen levels were near to what they are
now, approximately 500 million years ago.
However, even before the rise of oxygen, Earth was teeming with
microbial life -- this is the perspective that must be kept in mind when
searching for life on other planets of unknown evolutionary age. Indeed, other planets could be in any of
these stages, and the search for life can not simply assume that a given stage
of life or planetary evolution will have been reached. One should also keep in mind that the
evolution of Earth has been drastically impacted by life. The oxygen we breathe is a product of the
early evolution of photosynthesis, which supplies it. Without this innovation, the planet might well be alive, but its
life would look much different from what we see today.
Contributed by: Dr. Kenneth Nealson
|
