Evolutionary Timeline
No universe, no Earth, no humans.
So if you want to develop a timeline of human evolution, it starts with the universe.
And scientists know that it all started 13.8 billion years ago because they measured how fast space is expanding. That’s thanks to astronomer Edwin Hubble, who discovered a century ago that distant galaxies are moving away from us. (Probably a good thing they aren’t moving toward us and with Earth as the bullseye.) By running the film backward, so to speak, the scientists studied their stopwatches and calculated the “when” for that first big bang.
So, 13.8 billion years.
Give or take a million, perhaps.
The Earth came along 9.3 billion years later (4.5 billion years ago).
How do we know?
The rocks.
Earth rocks, asteroid rocks, moon rocks. They all line up. Radiometric dating makes it possible. Because unstable radioactive elements (like uranium) decay into stable elements (like lead) at an unchanging rate, it’s possible to calculate how long the decay has been happening.
On Carl Sagan’s Cosmic Calendar, as a reminder, if all of this history was one calendar year and the big bang happened at midnight on January 1, life would begin on Earth in mid-September, dinosaurs would appear on December 25, get wiped out on December 30, and modern humans would start walking the Earth in the final minutes of December 31.
Let’s break it down in a bit more detail and also highlight a handy timeline posted on a site called Brewtiful (we’ll take our handy timelines wherever we can get them):
3.5 billion years ago: Microscopic, single-celled organisms.
375 million years ago: Fish begin moving onto land
240 million years ago: Dinosaurs emerge and dominate for the next 160 million years.
66 million years ago: A large asteroid strikes near what is now Mexico.
7 million years ago: Early bipedalism among Hominins.
3.9 million years ago: Earliest stone tools.
3.3 million years ago: The genus Homo appears.
.9 million years ago: Neanderthals.
400,000 years ago: Homo sapiens.
300,000 years ago: Global migration.
70,000 years ago: Agriculture.
But, think about it.
In the big sweep of the history of the universe, the anatomical changes in the physical form of the human being all took place in a relatively short period of time. Check that—a very, very short period..
Those changes, all in the last little blip of time, include the fact the size of the human brain tripled in size after bipedalism began. This growth allowed for advanced tool making, abstract thought, and a little thing called language.
These changes include the fact that our faces became flatter and less projecting over time. Why? Softer, cooked foods required less heavy chewing power. (And one side effect is that modern humans have too many teeth and/or impacted wisdom teeth.)
Those changes include hands that evolved with shorter fingers and longer, more opposable thumbs. Our wrists and palm structures changed to support high-force gripping with improved motor control (important for chipping and shaping complex stone tools).
Those changes include hair loss and adaptations in our ability to sweat. Body hair drastically decreased and we added millions of eccrine sweat glands across the skin, allowing us less-hairy creatures to cool down efficiently while running in hot climates. Suddenly (well, you know what we mean) we could run. And run.
And those changes included changes in the shape of the pelvis including a new birth canal shape to help babies with large heads make their appearance. However, this was an evolutionary compromise that balances the mechanics of walking upright against the requirement of birthing a large infant head.
A lot of things had to line up to make it possible for us to stand in line in Starbucks waiting on a double-shot latte while we check Instagram on our phones. It started with the universe and, way back when, and the process is ongoing today.
What was going on before the Big Bang? Well, time did not exist.
(That’s a head-hurter. We’ll leave that one for the physicists to discuss.)
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