Let's explore how life
has changed and diversified
since the emergence of organic molecules
in basic cells. One way to view this process
is through six mini-thresholds. A threshold is a sudden increase
in complexity that gives rise to new,
emergent properties, new qualities. Consider each mini-threshold
for yourself. Does it mark something
new and different, and if so, why? Our first mini-threshold
is photosynthesis. It's likely that the first
prokaryotes evolved deep in the ocean
in massive vents that provided them
with chemical and heat energy. About 3.5 billion years ago, some cells migrated
to the ocean's surface and evolved to use vastly more
abundant energy from the sun. The process they developed
to do this is called photosynthesis. It led to an energy bonanza and that enabled life to spread
to many more places. And since oxygen is created as
a byproduct of photosynthesis, huge numbers
of photosynthesizing prokaryotes over millions of years radically transformed
our atmosphere from one rich
in carbon dioxide to one richer in oxygen. Oxygen was poisonous
for many species so they died off, but new species emerged that could use oxygen's
amazing chemical energy. Mini-threshold two
is the emergence of eukaryotes about 2.5 billion years ago. These are more complex cells
whose DNA is locked up inside a special case
called the nucleus, which help protect and preserve
vital genetic information. Eukaryotes also contain
tiny organs called organelles. Like the organs in your body, they perform special functions
such as photosynthesis or processing oxygen. This meant that eukaryotes
could thrive in Earth's increasingly
oxygen-rich atmosphere while many prokaryotes perished. That's a pretty
important development since we are made entirely
of eukaryotic cells. Around one billion years ago, we crossed mini-threshold
number three, the introduction of the first
multicellular organisms. In the same way
specialized organelles came together to form
more complex eukaryotes, different eukaryotes
came together to form even more complex
life-forms. These organisms could contain
billions of cells, each with a different function,
but all sharing the same DNA so they work together. With networks of specialized
cells and cooperation, multicelled organisms
could respond to changes in the environment
in entirely new ways, further developing a key
survival trait of life that we call homeostasis. The development of brains
is mini-threshold four. Multicelled organisms needed a way to coordinate
all the activities going on inside them and this became the work
of special nerve cells. In some organisms, these cells
began to cluster at the head and down the spinal cord
to form the first brains. Organisms with brains could process
much more information and they could react to it
in even more complex ways which enabled richer and more
sophisticated activities like thinking and perhaps even consciousness. Mini-threshold number five
is when life moves from the ocean to land. From about 475 million
years ago, some multicellular organisms, beginning with plants and fungi, left the oceans for land. There was a great incentive; this new environment
was rich in new opportunities for organisms that could find
ways to survive. But that was a challenge,
though, as these organisms had
to develop special skins to avoid drying out, special ways
to breathe out of water, and new ways to reproduce. The modern equivalent
would be humans trying to live in space. Mammals, the forerunners
of you and me, account for mini-threshold
number six. The first animal to live on land may have been
like a modern lungfish, but other land animals
soon evolved such as amphibians. But these still needed to return
to the water to reproduce. Then came reptiles like crocodiles or dinosaurs. These developed large,
leathery eggs that could survive
well away from water. Then about 250 million
years ago, the first mammals
appeared on Earth, evolving from a branch
of reptiles that loosely resembled birds. Mammals are warm-blooded, they're furry,
and they don't lay eggs, and you and I are mammals. Does the appearance of mammals really count
as a mini-threshold, or is it just
because we are mammals that we think so? What would be other candidates
for mini-thresholds, by the way? The development of backbones? The ability to think? Or what about
the concept of family?