Life. But where did life come from. It's pretty much accepted now that, like the Bible said, the waters and the land brought it forth.
Back in 1953, Stanley Miller under the supervision of Harold Urey published the description of an experiment in which Miller mixed up what he figured Earth's early atmosphere was like and shot an electric discharge through it over several days. The result was a dark soup. From the mixture of inorganic gases came a solution that contained organic compounds necessary for life including amino acids (the building blocks of proteins) and nucleic acids (the units of which our genetic materials are made). Life could come from no life, but did it?
Organic materials have also been found in meteorites, reviving the fantastic sounding theory that life came from outer space, but if that's the case, where did that life come from?
Regardless, life was on Earth a billion years after it's birth. It wasn't life as we know it. The organisms were what we call "extremophiles". They resembled some of the kinds of life that live on the ocean bed around volcanic vents. They might have "eaten" sulfur. Although they created oxygen as part of their metabolism, oxygen was toxic to them, so how did they survive? Enter "Red Earth".
Early Earth resembled meteors and asteroids a lot more than the lush green and blue planet we see today. Without a lot of oxygen in the atmosphere, there was a lot more metallic elements, especially iron, in the crust. Iron rusts, taking oxygen out of the atmosphere. At one point our green and blue planet was black, and then it turned red as it rusted. Extremophiles could, then, survive.
The cells in our body are extremely organized with their own organs (we use the term "organelles") that do specific jobs like manufacture proteins, pump materials into and out of the cells, and provide structure. Our cells are called "eukaryotic", because "sometimes you feel like a nut". (The Greek word "karyos" means "nut" and it refers to the nucleus of the cell which is certainly a "good" - "eu" is Greek for "good" - nut). Bacteria and extremophiles are prokaryotes. They don't (and didn't) have a well defined nucleus. Their genetic material just floated around inside their cells (ours is stored in our cells' nuclei.) Early cells were just sacks of chemicals.
Fossils are the remains of past life that have typically been turned to stone by the precipitation of silica or calcite rich solution to replace their organic components. Most organisms just die and rot. Fossils are very rare and soft tissue very (very!) rarely becomes fossils. What we know of early extremophiles, we learn from the ones that exist today (for instance, in hot pools at Yellowstone National Park and at mid-oceanic rifts) and by a lot of painstaking deduction.
The oldest fossils of confirmed age are stromatolites, fossils of colonial organisms. The oldest seem to be cyanobacteria but most are eukaryotic organisms akin to our algae. The oldest confirmed fossil is 2.724 billion years old. The oldest fossils might be 3.5 to 4.1 billion years old.
By the time extant fossils formed, life had to exist long before. The earliest life forms never had the chance.
The geological record is divided into four eons. The Hadean eon began with the Earth's formation and ended 4 billion years ago. The standard unit of geologic time is abbreviated "Ma" and stands for a million years. Geologists also use Ga to represent a billion (giga-) years. We generally talk about "years ago".
The Archean eon started where the Hadean left off and ended 2,500 Ma ago. This is roughly before life on Earth, but there seems to have been some very primitive life forms.
Life really took off during the Proterozoic eon, which ended 538.8 million years ago. This was the time from the appearance of an oxygen rich atmosphere to the rise of complex life forms. "Proterozoic" literally means "early life".
We live in the Phanerozoic eon. The plot of land that will be Colorado has not existed up to this eon. The Greek "phaneros" means "visible" so this is the time of complex life forms, life that can be seen without the aid of a microscope.
The longest segment of time in the geologic time scale was the Proterozoic. Life was on Earth surprisingly quickly but it took a long time for it to really take off.
The transition from prokaryotes to eukaryotes was an odd event. Prokaryotic cells ingested other prokaryotes sorta like amoeba or The Blob, and the food lived! They became organelles. Cells differentiated and organized and then they started living together in colonies. Then different parts of the colonies became specialized for different life functions and complex life forms appeared.
And, eventually, there was Colorado. How did that happen?
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