Although the exact process by which life formed on Earth is not well understood, the
origin of life requires the presence of carbon-based molecules, liquid water and an energy
source. Because some Near-Earth Objects contain carbon-based molecules and water ice, collisions
of these object with Earth
have significant agents of biologic as well as geologic change.
For the first billion years of Earth's existence, the formation of life was prevented by a
fusillade of comet and asteroid impacts that rendered the Earth's surface too hot to allow
the existence of sufficient quantities of water and carbon-based molecules. Life on Earth
began at the end of this period called the late heavy bombardment, some 3.8 billion years
ago. The earliest known fossils on Earth date from 3.5 billion years ago and there is
evidence that biological activity took place even earlier - just at the end of the period of
late heavy bombardment. So the window when life began was very short. As soon as life
could have formed on our planet, it did. But if life formed so quickly on Earth and there
was little in the way of water and carbon-based molecules on the Earth's surface, then
how were these building blocks of life delivered to the Earth's surface so quickly? The
answer may involve the collision of comets and asteroids with the Earth, since these objects contain abundant
supplies of both water and carbon-based molecules.
Once the early rain of comets and asteroids upon the Earth subsided somewhat,
subsequent impacts may well have delivered the water and carbon-based
molecules to the Earth's surface - thus providing the building blocks of life itself. It
seems possible that the origin of life on the Earth's surface could have been first
prevented by an enormous flux of impacting comets and asteroids, then a much less
intense rain of comets may have deposited the very materials that allowed life to form
some 3.5 - 3.8 billion years ago.
Comets have this peculiar duality whereby they first brought the building blocks of life to
Earth some 3.8 billion years ago and subsequent cometary collisions may have wiped out
many of the developing life forms, allowing only the most adaptable species to evolve
further. It now seems likely that a comet or asteroid struck near the Yucatan peninsula in Mexico
some 65 million years ago and caused a massive extinction of more than 75% of the
Earth's living organisms, including the dinosaurs. At the time, the mammals were small
burrowing creatures that seemed to survive the catastrophic impact without too much
difficulty. Because many of their larger competitors were destroyed, these mammals
flourished. Since we humans evolved from these primitive mammals, we may owe our
current preeminence atop Earth's food chain to collisions of comets and asteroids with
the Earth.
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