Lecture 48: Big History—Humans in the Cosmos
Africa. What made them different from all other animals, and enabled them
to explore so many different environments, was their remarkable ability to
exchange and store information about their environments. Humans could
talk to each other, they could tell stories, and unlike any other animals, they
could ask about the meaning of existence! Their appearance counts as a
fundamental threshold in our story.To understand how the ¿ rst humans appeared, we must survey the history
of life on Earth. Like all other species, our ancestors evolved by natural
selection. They evolved from intelligent, bipedal, ape-like ancestors known as
“hominines” that had appeared about 6 million years earlier. The hominines
were descended from primates: tree-dwelling mammals with large brains
and dexterous hands that had ¿ rst appeared about 65 million years ago.The mammals were furry, warm-blooded animals that had ¿ rst evolved
about 250 million years ago. They were descended from large creatures
with backbones, whose ancestors had left the seas to live on the land
about 400 million years ago. These were descended from the ¿ rst multi-
celled organisms, which appeared about 600 million years ago in the
Cambrian era.During the preceding 3 billion years, all living organisms on Earth were
single-celled. The ¿ rst living organisms had appeared by about 3.8 billion
years ago, just 700 million years after the formation of our Earth. They were
the ancestors of all living creatures on today’s Earth. The speed with which
they appeared suggests that life is likely to appear wherever there are planets
bathed in the light and energy of nearby stars but far enough away for liquid
water to form. The appearance of life is one of the most important thresholds
in the big history story.Life could evolve only after the crossing of three earlier thresholds: the
creation of planets, stars, and chemical elements. Our Earth was formed
about 4.5 billion years ago, along with all the other planets, moons, and
asteroids and comets of our solar system, from the debris formed as our Sun
was created. Solar systems probably formed countless billions of times in
the history of the Universe.