Geoscience Reference
In-Depth Information
hardly wait to scramble out of our vehicles and begin prospecting. But
once we had discovered the fossils, we also knew that these breath-
taking outcrops also contained the key evidence for interpreting
when the embryos died.
It is difficult for human beings to appreciate how long ago the ani-
mals preserved as fossils at Auca Mahuevo lived. The average life span
of a person living in the United States is about seventy-five years. The
United States itself is slightly more than two hundred years old. The
earliest human civilizations based on agriculture from which we have
discovered artifacts existed about ten thousand years ago. The Ice
Ages ended about twelve thousand years ago, when such animals as
saber-toothed cats, mammoths, mastodons, glyptodonts, and giant
ground sloths went extinct. The earliest members of our human
species lived around one hundred thousand years ago, and our earli-
est human relatives first walked the earth about 4.5 million years ago.
All the dinosaurs, excluding birds, died out 65 million years ago. But
the animals living at Auca Mahuevo lived millions of years before
that. Based on previous studies of fossil animals that had been col-
lected from the same layers of rock that were exposed in other parts
of Patagonia, we knew that the rocks at Auca Mahuevo were
deposited sometime between 70 million and 90 million years ago, a
nearly incomprehensible span of time.
Geologists and paleontologists have developed a special time scale
to serve as a geologic and evolutionary calendar. Based on major
changes in the kinds of fossil organisms that lived at different times
during the history of the earth, this time scale is divided into four
major eras. From oldest to youngest, they are called the Precambrian
(4.5 billion to 570 million years ago), the Paleozoic (570 million to
250 million years ago), the Mesozoic (250 million to 65 million years
ago), and the era we now live in, the Cenozoic (65 million years ago
to the present).
The earth first formed slightly over 4.5 billion years ago at the start
of the Precambrian era, as the planets consolidated from rings of star
dust orbiting our sun. The earliest fossils of ancient life that we have
found were simple, single-celled organisms related to modern blue-
green algae, which lived in Precambrian oceans about 3.8 billion
years ago.
The first of our vertebrate relatives did not appear until almost 3.3
