microbes are our ancestors
In the beginning there were microbes.
Microbes have literally created - and are continuing to create - the world as we know it. They make the world go round - by driving the planetary cycles of carbon, nitrogen, sulfur, and phosphorus. Microbes convert these elements into compounds that can be used by plants and animals and then return them to the earth through decomposition. Microbes were the first organisms to make their own food, through photosynthesis. They released oxygen, creating the atmosphere as we know it. In fact, it's photosynthetic bacteria in the ocean that produce more than half the oxygen we are currently breathing.
I'm reading a fascinating book right now called "I Contain Multitudes: the microbes within us and a grander view of life" by Ed Yong. He is a journalist and science writer, and I really appreciate him for taking super sciency-technical stuff and breaking it down for the lay person to digest.
So the earth is 4.54 billion years old. If we were to imagine collapsing that amount of time into a single calendar year, right now it would be just about to strike midnight on December 31st. We humans have been around for about 30 minutes. Plants and animals and all multicellular living things have only been around since October. From March until October of this hypothetical year, microbes were the only living things on earth.
We evolved from microbes.
Before there were multicellular creatures of any kind on earth, there were only two types of living things: bacteria and archaea. Both of these are single-celled organisms without any kind of structure to their cell - no nucleus or anything like that. Bacteria and archaea pretty much just floated around doing their own thing for the first 2.5 billion years of life on earth. Then one day - against all odds - a bacterium merged with an archaean, and the two became one.
In my mind, it looks like this:
Maybe a little less like they were eating each other, and more like their membranes were fusing, but you get the idea.
The merger itself is not so improbable, that may have happened before and since, but the super improbable thing is that the bacterium survived in there. That's not really a thing that happens. In fact, it has never happened before or since this one time, in 4.54 billion years.
Yet somehow they made it work.
The archaean accepted the bacterium into itself, made space for it, built infrastructure to take care of its needs as it reproduced and made more little bacteria. And these particular bacteria bring a special power to the relationship. They produce electricity. Actually really a lot of electricity, and they contributed this energy to the host cell. They became what we now know as mitochondria - remember those guys from biology class? They look like little kidney beans with a scrunched up intestine inside them - we call them the "powerhouse" of the cell.
These domesticated bacteria changed everything - by providing an extra source of energy, they allowed cells to get bigger (like 10,000x bigger), to copy more genes and to become more complex, giving rise all the creatures that we see around us today, including you and I.
And we know this because our own DNA - and the genomes of all creatures - still contain genes from our original ancestors - the bacteria and archaea.
All life as we know it came from this one unlikely union.
Thanks for making it work, little microbes.
Here's a great radiolab episode about exactly this topic, I highly recommend it.