The Earth was created approximately 4.6
billion years ago when a nearby star imploded
and dust, rocks and gas in the solar system
collided and, because the temperature was so
high, the material was effectively glued
together. Over time, more and more material
(rocks and dust, mostly) collected together and
they became larger and larger objects that
ultimately made up a planet. Life began on Earth
somewhere between 4.4 billion years ago (when
water vapor first formed) and 2.7 billion years
ago (where we have the first evidence of
photosynthesis). Scientists are still working to
understand exactly how the first life forms were
formed, but most early life scientists agree
that early life forms were most likely small,
simply bacteria, containing a single cell. Each
cell has within it a "code" that records what it
is made of and the recipe for how to make itself
again. Sometimes, this record got disturbed and
the recipe wasn't exactly right, so instead of
making 1 cell, the recipe made 2 cells that were
joined. Through many "mistakes" in the recipe
over time (something scientists now call
evolution), single-celled organisms evolved into
cooperative teams of cells that would replicate
themselves. Almost every scientist believes that
humans have evolved over billions of years
through simple changes in the "recipe."
have attached a diagram of the "Geologic Clock,"
which shows the relative time it took for all
these changes to happen.
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