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The first known organisms were single-cell bacteria called prokaryotes, which means their genetic material (DNA) is not separated from the rest of the cell by a nucleus.
Because oxygen was absent from Earth’s early atmosphere and oceans, the first organisms employed anaerobic (without oxygen) metabolism to extract energy from “food.”
Their food source was likely organic molecules in their surroundings, but the supply was very limited.
Later, bacteria evolved that used solar energy to synthesize organic com pounds (sugars). This event was an important turning point in evolution—for the first time organisms had the capability of producing food for themselves as well as for other life-forms.
Rise of oxygen:
Photosynthesis by ancient cyanobacteria, a type of prokaryote,
contributed to the gradual rise in the level of oxygen, first in the ocean and later in the atmosphere.
Thus, these early organisms dramatically transformed our planet. Fossil evidence for the existence of these microscopic bacteria includes distinctively layered mounds of calcium carbonate, called stromatolites
Stromatolites are limestone mats built up by lime-accreting bacteria.
What is known about these ancient fossils comes mainly from modern stromatolites like those found in Shark Bay, Australia
The oldest fossils of more advanced organisms, called eukaryotes, are about 2.1 billion years old. The first eukaryotes were microscopic, water-dwelling organisms. Unlike prokaryotes, the cellular structures of eukaryotes contain nuclei.
This distinctive structure is what all multi-cellular organisms that now inhabit our planet—trees, birds, fishes, reptiles, and humans—have in common.
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