Early Life: Our Best Understanding
Deep in the past, something began to stir;
we may never understand, how that strange thing occurred.
Talking about the earliest life on Earth is hard. We know it existed. We know it must have been very simple and probably in the ocean. But we have so little evidence about it, that anything more is difficult to confirm. And the whole question has an underlying problem: what does it mean to be alive? Let’s dive in, to the earliest life on Earth.
What is life? When is something alive?
When we think of “life”, we generally think of big animals and plants – lions, elephants, and trees. It’s also pretty clear that bacteria are alive – they move around, they eat stuff, they reproduce, even if they are quite different. But what about an atom? Is an atom alive? They can move around? They can sort of reproduce, by combining together with other atoms?
Atoms are not alive. They don’t eat, or die, and their behavior is reasonably predictable. But once you get a bunch of atoms together to form a molecule, they behaving a lot more randomly. Also, if they have the right materials around them (like in a nice wet and warm test tube) those molecules can often reproduce and suddenly make more of themselves. But, they will generally only do it in certain environments – like the right amount of light and heat. And if they don’t have those conditions, if you freeze or overheat them, they can kinda “die” by changing their form into some new chemicals that won’t be able to reproduce. Does this mean some molecules are alive?
Molecules are not alive either. Their behavior is still too simple and predictable. However, it is definitely true that the simplest organisms are often closer to bundles of molecules than they are to the animals and plants that we know. The first alive things on earth were probably just some very simple chemical reactions that started to reproduce themselves. Then, some of these chemical processes probably combined together, getting a bit more complex. Eventually, it turned into something complicated enough, that could respond to light and heat by changing the chemical processes. Eventually, it became alive.
To give an example of all this – we have gone some way to recreating the creation of life in a lab. One particular famous experiment is called the “Miller Urey” experiment. The design was very simple, two researchers Miller and Urey put some very simple chemicals which commonly occurred in the Earth’s early oceans (water, methane, and hydrogen) in a large glass jar of water and sealed it. They then heated the glass and ran sparks through it to simulate lightning and give the system some energy. They then let it sit for a few weeks, and broke into the container, and investigated what had happened. They found that large numbers of basic chemicals that are required to create life, had just formed themselves in the jar. The experiment demonstrates that it is possible for biological, life-like compounds to basically create themselves under the right conditions.
What did the first life look like?
The first life on Earth would have been very simple. It would not have been something complex like a plant or animal. It would not have been multi-cellular, instead, it would be closer to something like bacteria. And frankly, it would have been a lot simpler than most bacteria, some of them are still pretty complex. We are looking at very simple organic machines.
The earliest life on Earth almost certainly was not green (see our post on the origins of photosynthesis). A lot of the most simple life on Earth currently uses the sun to create energy in a process called “photosynthesis”. However, this requires quite complex chemistry which took a while to evolve. It would not have just popped out of nowhere.
Another point to note – all life on earth has a surprising number of similarities at the chemical level. At the most basic level, organisms on Earth today have DNA and are constructed of the same basic chemical building blogs called “amino acids”. This is actually really amazing and a lot of people take it for granted. When you look at a tiger and a tree, despite how different they are, at a fundamental level, the building blocks that make them up are the same. It’s very easy to imagine a different world, where you had a few different types of life that were quite separate (maybe one made mostly out of rocks, another out of metal etc). But we don’t on Earth. The legobricks that make up all life forms are basically the same.
The oldest life that is still around today is called “Archaea”. They are our best guess about what these first organisms looked like. We still don’t understand what these things are and they have only been discovered in the last 40 years or so. They were initially just thought to be weird bacteria because they look very similar – they have cells and DNA. However, they tend to live in really extreme environments like hot springs and can often be chemically quite different from bacteria.
Where did the first life live?
We don’t know where on Earth life started. It happened so long ago that the Earth’s continents have been rearranged several times since then. We’ll never be able to say something like, it evolved in Asia or Europe or anything like that.
It’s pretty likely that life started in water. Because the earliest life on Earth formed from simple chemical reactions the entire process was a little bit of a numbers game, combining chemicals until something exciting happened. The place where you had the most opportunity for chemicals to combine is water. For similar reasons, it’s likely life occurred somewhere warm. The warmer a place is and the more energy it has, the more chemical reactions can occur. Chemistry occurs much slower in cold places.
Other than somewhere wet and warm, we don’t really know where life on Earth started. But we have a number of very interesting guesses.
One potential place life may have started on Earth are deep-sea hydrothermal vents. Deep under the ocean, there are holes in the bottom of the Earth’s crust where lava pours out. They are very warm, often above 100 degrees celsius, but the pressure is so high that the water can’t boil. Even in the modern age, these are places of very exciting chemistry and they have extremely weird and unusual organisms living on them, feeding off the heat of these vents which are completely separate from the outside world.
We have a variety of other weird and wonderful theories. For example, some researchers think radioactive material being thrown around on the early earth might have played a role, creating something like a naturally occurring nuclear reactor. Others have theories about volcanic eruptions sending large amounts of volcanic ash into the ocean because of the weird shapes it makes at a molecular level. Others even think it is possible life emerged on another planet and was taken here by asteroids. They are all glorious options. But regardless, it was a weird and wonderful thing, that we still don’t really understand.
