The probability of us being here is almost identical to Zero. We result from a number of very unlikely events scattered over 4-1/2 billion years, none of which has happened more than once. If any one of them took 20% longer than it did, we would have missed the habitable Earth.
In Darwin's day, evolution was thought to be a very slow steady process. Today we know it happens very quickly, usually in an isolated population that is under extreme stress. That's why those "missing links" are so hard to find. They barely existed.
This Earth formed a little over 4.5 billion years ago. That's 4,500 million years. This formation was hot and violent with a stew of corrosive chemicals and constant bombardment. The whole surface was molten magma, yet less than 100 million years later, it was cool enough that water began to accumulate.
In only about 200 million years from the Earth's formation, the first simple life forms appeared in the hot and turbulent chemical stew. How this happened so quickly is still unknown. Those toxic and corrosive chemicals and high temperatures seem to have played an important part, particularly cyanide.
Some time between the 4.3 billion and 3.4 billion years ago emerged a simple life form we call LUCA, "Last Universal Common Ancestor". Every living thing on Earth, is descended from LUCA.
By 3.5 billion years ago microscopic bacteria had evolved, and they were the only life for the next 1-1/2 billion years - or so we thought. Then, in 1977, it was realize that about half of those supposed Bacteria were actually Archaea. This is important because we are directly descended from Archaea.
During these times Earth had an atmosphere of Methane, Ammonia, Carbon Dioxide, Carbon Monoxide, Nitrogen, Hydrogen Cyanide, and other noxious gasses, but no Oxygen.
During this 1-1/2 billion years, a wildly successful Bacteria we call Cyanobacteria learned how to use sunlight to break Carbon Dioxide into Carbon and Oxygen. It used the carbon to build more of itself, and discarded the Oxygen. All this Oxygen caused a period called "The Great Rust".
Finally, there was nothing left to rust, and Oxygen began to accumulate in the atmosphere. Most life on Earth died from this poison, but some Bacteria and some Archaea were able to adapt and life continued.
Then a little over 2 billion years ago a very unlikely event occurred. Some Archaea got its DNA tangled up with that of a Bacteria, and survived, and multiplied. These chimeras went totally off the rails, becoming 100 times the size and 8000 times the volume of a bacteria, and their interior workings started getting really complicated, including their reproductive cycle. These critters are called Eukaryotes.
Somewhere along this time, a Eukaryote cell was invaded by a Bacteria, which found these huge cells comfortable and nutritious, and stayed. It evolved with the cells, but remained an alien with it's own DNA, and it paid rent.
Today, your body is a community of about 7 trillion Eukaryote Cells, and every one is still infested with hundreds to thousands of these invasive aliens. The mother's egg is already infested before fertalization. They are called Mitochondria, and without them we would be dead in a minute. They convert low grade energy brought by the blood to high grade energy, enabling our active lifestyle.
Some other Eukaryote cells, already infested with mitochondria, also became infested with an Oxygen producing Cyanobacteria. Thus was the division between Animals and Plants. Those invaders, also with their own DNA, evolved into Chloroplasts, and continued splitting Carbon from Oxygen.
Of course, the Animal Eukaryotes couldn't generate their own energy, so they took up eating plants. Some others took up eating Plant Eating animals. Plants in turn absorbed nutrients from dead animals and animal waste. Interrelationships started to be really complex.
Then, about 1-1/2 billion years ago, Eukaryotes of both lines started to stick together into primitive communities. But evolution was still slow and halting. Then, only 1 billion years ago, came the evolutionary Big Bang. Both the Animal and Plant lines developed sexual reproduction, and evolution took off like Gang Busters. More than 99% of evolution has happened in the last billion years.
Around 960 million years ago the Animal line, the Opisthokonts, split into two closely related but very different lines, Animals and Fungus. The reason fungal infections are so difficult to treat is this close relationship. Poison for one is poison for the other.
I keep telling Vegans they can't eat mushrooms because they're animals. There were other splits as well, the best known is the seaweed Kelp, which is neither plant nor animal, but more closely related to us than to plants.
Now we are at less than 500 million years ago. The seas are filled with plants and animals of all descriptions, but the land is totally barren. Plants absorb needed minerals from the sea, but on land minerals were all locked into rocks. There was no soil. Plants couldn't live there.
Fungus could break down rock into the minerals plants needed. Fungus needed nutrients plants could provide. Together, they conquered the land - and runoff from the land fertilized the seas.
The plant / fungus relationship is still very strong, with certain fungi associated with certain plants, which grow much better if that fungus is present.
Next came the animals. There were now vegetables to eat. First came the microbes, then insects and spiders. This was a wonderland, lacking the voracious predators of the seas. Cockroaches grew to 3 feet long! But this was not to last.
There came a fish. Not the modern fish you eat for dinner, but an ancient evolutionary side branch called "lobe finned fish". They had both pectoral and pelvic girdles. The four fins attached to those were on stalks of jointed bones, with bony fin rays. They also had rudimentary lungs. These critters were well suited to invade the land, and they did. We know these fish well, because they are still with us.
Living in the unstable environment of shallow waters and swamps, some evolved very quickly. In only 20 million years they went from shallow water fish into land living critters called tetrapods. Those tasty 3 foot cockroaches soon had to get real small, learn to run fast, and hide under the stove.
Just 318 million years ago, two lines of tetrapods emerged, Diapsids (that's reptiles) and Synapsids (that's us). For millions of years we ruled the world. In dinosaur pictures you often see a large fierce lizzard like critter with a big fin on his back. That isn't a dinosaur, dinosaurs didn't exist yet, it's a Synapsid. That fin was an early form of temperature control.
Then, During the "Great Dying", 281 million years ago, nearly all the Synapsids perished, along with at least 80% of all life on Earth. This was caused by a huge lava flow in Siberia that still covers 3 million square miles and is 1 million cubic miles in volume. Our kind barely squeaked through, and Diapsid Dinosaurs took control.
We Synapsids had to evolve into small, furry, warm blooded mammals living in burrows. We came out mostly at night, but badger size ones are now known to have killed and eaten much larger plant eating dinosaurs.
Then, 66 million years ago, the Earth was hit by a double whammy. On one side of the world, a huge astroid hit in the Gulf of Mexico and set half the world on fire. On the other side, In India, a huge lava flow covered about 1,500,000 square miles up to 2 miles thick, poisoning the atmosphere and blocking the sun. The Dinosaurs were wiped out, except birds. In the recovery, Synapsid mammals emerged from their burrows to reclaim the world.
Mammals quickly evolved in many directions. One line produced the apes, which were adapted to forest and jungle. Our line is thought to have diverged from the Chimpanzees around 8 million years ago and adapted to a savana environment.
I'm not going to detail human evolution. The neat straight line of evolution we learned in school has been replaced by a tangled mess, and everything is under intense debate. It's not even "out of Africa now", its back and forth, and not just once. And that bulky hunchback cave man still seen in cartoons never existed.
I will mention Homo Erectus, the first in our line that looked very much like us. This species appeared 2 million years ago and was very successful, living for almost 2 million years, well into the age of his modern human decendents. His range was from the West coast of Africa to Korea. Homo Erectus was the first to leave vegetarianism and become a hunter of large animals. Most important, he learned to control fire and cook food. The resulting increase in safety and nutrition contributed much to our development.
Now we stand on the brink of another great extinction, caused by our own success. This can no longer be stopped. We could mitigate it somewhat, but probably won't - see Trump administration. This will result in rising temperatures and extremely wild weather which we are already starting to see. Large regions of the world will become uninhabitable, famine will be severe, and human populations will decline amid great suffering. A lot of Florida will be under water and the rest subject to impressive hurricanes.
Of course, we could also have a huge lava flow event. There are two supervolcanos under the US, but the most likely to erupt is under Japan. would wipe out all life there and severely affect the rest of the world. Of course, the resulting volcanic winter would mitigate global warming for a century or more.
Given how unlikely intelligent life is here, expect it to be very scarce in our galaxy. Actually it's a lot worse than that. Now that we can study other solar systems, it is clear that in their formation, the outer gas giant planets normally migrate towards their sun, wiping out any planets in the liveable zone. That was going to happen here, but some unusual gravitational interaction caused Jupiter to change direction. So we are here, and ET isn't.
Andrew Grygus
Life01 250323