Chapter 18 Fossils and Geologic Time

Rock fragments from Mars have been found in Antarctica.
These rocks provide another clue to the ability of life to exist
in extreme environments. It appears that the impact of objects
from space, such as meteorites, blasted rock fragments
from the surface of Mars. Some of these rocks fell to Earth.
Microscopic examination of these rocks has revealed tiny objects
that many scientists believe to be evidence of life. Could
very simple forms of life travel from planet to planet as
spores in rock fragments like these? While this is an interesting
possibility, scientists do not know for sure. Further investigations
are needed to help scientists decide how life on
Earth began.
Very few rocks survive from the earliest part of the
planet’s history. Furthermore, metamorphism has drastically
changed most of the oldest sedimentary and igneous rocks.
Some scientists think that they have found evidence of carbon
compounds that must have been created by living organisms
nearly 4 billion years ago. This evidence is controversial.
Stronger evidence is provided by patterns of life structures.
The oldest recognizable fossils are probably clumps of primitive
bacteria called stromatolites. These fossils date from
about 3.5 billion years ago. Similar clusters of bacteria are
alive today in Western Australia.
WHAT IS ORGANIC EVOLUTION?
WHAT IS ORGANIC EVOLUTION? 419
The oldest fossils are remains of tiny single-celled organisms.
These life-forms were so primitive that their cells did
not contain a nucleus. They reproduced by splitting into two
or more new cells. The first critical advance in the development
of more complex life-forms is found in rocks about 1.4
billion years old. Cells with a nucleus appeared. These cells
could reproduce sexually and inherit characteristics from two
parent cells.
Colonies of single-celled organisms developed into simple
multicellular organisms, such as jellyfish and worms, less
than a billion years ago. The first organisms with shells and