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Life on Earth has changed a lot since it first began. Take a journey through time with David Christian to explore the mini-thresholds of life.
Think about the following questions as you watch the video
What makes the emergence of photosynthesizing cells a mini-threshold?
What makes the emergence of eukaryotes a mini-threshold?
What makes the emergence of multicelled organisms a mini-threshold?
What makes the emergence of brains a mini-threshold?
What makes the move of sea creatures to land a mini-threshold?
What makes the emergence of mammals a mini-threshold?
: 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?
