Remember what the biggest questions is that we ask ourselves as scientists: where did we come
from, and how did we get here?
Well, here it is, folks! The last part in my timeline for the natural history of the Universe. In part 1
, we talked about what happened during the first 50 million years, from inflation to the formation of the first stars. In part 2
, we went from the formation of the first galaxies to the formation of our Sun.
I now bring you part 3, from our Solar System to the evolution of the earliest humans. Let’s starts with a brief recap of the first two parts, and then let’s get right to it!
- 10-35 seconds: Inflation — stretching the Universe flat and smooth.
- 10-30 seconds: The Big Bang — filling the Universe with matter, antimatter, and radiation.
- 10-30 to 10-10 seconds: Baryogenesis — creation of more matter than anti-matter.
- New! 10-5 seconds: Confinement — the first protons and neutrons form.
- 3 minutes: Big Bang Nucleosynthesis — creates the nuclei of the light elements.
- 380,000 years: Recombination — creates the first neutral atoms and emits the cosmic microwave background.
- 50 million years: The Universe forms the first stars.
- 300-500 million years: The earliest galaxies begin to take shape.
- 900-950 million years: Reionization — the Universe becomes transparent to radiation as the neutral intergalactic atoms finally become ionized.
- 3 billion years: Galaxy Clusters form, clumping together in groups of hundreds or even thousands.
- 3.5 billion years: The Universe’s expansion rate changes, allowing us to first see the evidence for dark energy.
- 9.1-9.2 billion years: A dust cloud in a spiral arm of the Milky Way collapses to ignite nuclear fusion, forming our Sun. Welcome to part 3!
- 9.2 billion years: As the Sun is forming, a thin disk of matter remains around it, known as a proto-planetary disk.
This matter eventually gets swept up into little balls, and forms the planets we know and love. Of the four inner planets, Earth is special; there were two relatively large balls (one about the size of Venus, the other about the size of Mars) that smacked into each other early on.
After the collision, the big glob left over formed the Earth, and the debris that was kicked up eventually was formed, by gravity, into the Moon.
- 9.5 billion years: Dark Energy, the mysterious force driving today’s expansion of the Universe, becomes more important than matter in causing the Universe to expand.
- 9.7 billion years: The oldest rocks on Earth were formed. The rock is known as the Acasta Gneiss and can be dated by the zircons inside of it. It’s located in the Northwest Territory of Canada, and you can actually go there any time you like; just plan a trip to Slave Craton. The Acasta Gneiss looks like this:
- 9.9 billion years: The first life on Earth appears! We learn this from looking at very old rocks in Greenland, along the Isua Greenstone Belt. How do we know there was life just from looking at the rocks? Carbon abundances. Where there’s life (or biomass), there’s a different ratio of Carbon-13 to Carbon-12 than where there’s just plain ol’ rock. And these rocks, which we can date to be 3.8 billion years old, show a difference of a factor of 100 trillion in their Carbon abundances. That is some pretty strong evidence!
- 10.7 billion years: Mars dies! Sorry, all you fans of the Red Planet, who were rooting for life on Mars to do just as well as life on Earth. Despite possibly having oceans, life, and a thick atmosphere just like Earth, this is the end of the road for Mars. Because it’s so little, only 11% the mass of Earth, Mars’ core cools and ceases to be active, killing its magnetic field. This allows the Sun to strip Mars’ atmosphere away, causing the oceans to either freeze or boil, and ending the road for life on Mars.
- 12.5 billion years: Two huge leaps happen in evolution. First, Red Algae, the first multicellular organism evolves on Earth! (It only took 2.6 billion years.)
And second, the first organisms with two different genders evolve! This step is huge, because sexual reproduction allows evolution to happen much faster than plain ol’ dividing and making a copy of yourself.
- 13.2 billion years: The Cambrian Explosion happens! After very few really interesting, large living things coming to be on Earth for the first 4 billion years, the Earth suddenly finds itself awash in life. Everything from starfish to crabs to boneless fish to worms, mollusks, and insects appears. And mixed in with all of that, one very special fish evolved: the myllokunmingia. This fish is special because it’s the first form of life to ever evolve with a spinal cord. Almost all of the advanced animals, including every amphibian, reptile, dinosaur, bird, and mammal, evolved from this fish. Here is its fossilized remains:
- 13.5 billion years: Pangaea breaks up! All the land on Earth was once united in one supercontinent, but volcanic activity deep inside the Earth keep things on this planet moving, and caused this land mass to break apart and drift. It was just a short 200 or so million years ago that they were joined together.
- 13.65 billion years: The dinosaurs go extinct! After a great asteroid impacts the Earth.
We can see the evidence everywhere we look, because there is a separate geologic layer that’s a telltale sign of a worldwide catastrophe.
Once those large predators are gone, these little tiny birds and mammals are free to inherit the Earth. And what did they do in not even a hundred million years?
- 13.72 billion years: They evolved the first hominids, giving rise to our favorite great apes, us!