History #9: Life

This is a continuation of an ongoing series on big history. It follows the structure of the course developed by historian David Christian. This post will explore the origins of life.

Past posts can be found here: The Big Bang, Star Formation, Solar Systems

The story so far

(adjusted scale: all of time condensed into 13 years)

About 13 years ago, unknown conditions set off the Big Bang. This produced space, time, energy, and matter (mainly hydrogen and helium). The universe was born.

About 36 days later, cooling and tiny variations in the density of matter allowed gravity to bring together hydrogen and helium into high pressure clumps. Stars were born.

High temperatures within those stars, along with supernova explosions, allowed for the synthesis every other element on the periodic table. Chemistry was born.

As new stars formed around chemically rich clouds, solar systems were born. About 4.3 years ago the Earth was formed.


As we talked about last week, planets provided the universe with an assortment of complex chemical compounds. With the right amount of energy and liquid water, these compounds could combine in new and interesting ways. Energy was provided by the sun; enough to fuel chemical reactions but not so much as to blast molecules apart. Being the right distance from the sun also allowed for liquids to form. In gases, molecules are too far apart to link up. In solids, molecules can’t move very much. But in liquids, they are free to move and interact. All of this is referred to as the Goldilocks conditions for life.

Life can be defined, more or less, as anything with the following qualities:

  1. Metabolism: the ability to take in energy from the environment in order to power itself.
  2. Homeostasis: the ability to self-regulate internal systems in order to maintain stability
  3. Reproduction: the ability to make copies of oneself
  4. Adaptation: the ability to change form generation to generation

Brief timeline of life

3.8b yrs ago: Prokaryotes may have evolved deep in the ocean near massive vents that seeded the ocean floor with energy and chemicals. As they migrated towards the surface, they evolved the ability to use the energy from the sun in a process called photosynthesis. A byproduct of photosynthesis is oxygen, and over the years our atmosphere was transformed.

2.5B yrs ago: Eukaryotes were more complex versions of prokaryotes. They protected their DNA in a nucleus and had organelles to perform functions around the cell. One function was the processing of oxygen which allowed eukaryotes to thrive in the new oxygen rich atmosphere. Prokaryotes were less successful.

1.5B yrs ago: In the same way organelles came together to form eukaryotes, eukaryotes came together to form multi-celled organisms. The cells would work together to keep new organisms in homeostasis.

0.5B yrs ago: Multi-celled organisms developed nerve cells as a way to coordinate all the activities going on inside them. In some organisms these cells clustered around the head and spinal cord. The first brains were formed.

475M yrs ago: Plants and fungi left the oceans for land. Amphibians and reptiles soon followed. Skins and new ways of breathing were developed.

250M yrs ago: The first mammals evolved from a branch of reptiles that loosely related birds. Mammals were worm blooded and didn’t lay eggs. We are mammals.


Much like the Big Bang, not much is known about the origin of life. This post suggested that ocean vents were the starting point. However, other theories claim that life originated on other planets and was brought to earth by an asteroid. Evidence is still being gathered to support competing hypothesis and it is possible the this question will be answered in our lifetime.

I’ll leave you with this thought. Over the course of 13 billion years, the universe went from the most complex structure being a star to the most complex structure being a brain. The brain has 100 billion neurons, each being connected to 10,000 other neurons. There are more connections in the human brain than there are stars in the galaxy.




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