Yes, life existed 2 billion years ago, and it was likely quite diverse, although predominantly microbial.
While definitive, visually complex fossil evidence from 2 billion years ago can be scarce, several lines of evidence confirm the presence of life:
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Microbial Fossils: Although not always easily visible to the naked eye, fossilized microorganisms have been found in rocks dating back billions of years. These are traces of ancient bacteria and archaea.
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Stromatolites: These layered sedimentary structures are formed by microbial mats, particularly cyanobacteria (blue-green algae). Stromatolites provide physical evidence of microbial communities existing and thriving. Some of the oldest stromatolites date back over 3.5 billion years, indicating that microbial life was well-established by 2 billion years ago.
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Banded Iron Formations (BIFs): These are sedimentary rocks composed of alternating layers of iron oxides and chert. Their formation is strongly linked to the presence of photosynthetic organisms, specifically cyanobacteria, which released oxygen into the oceans. The oxygen reacted with dissolved iron, causing it to precipitate out and form the iron oxide layers. BIFs are found in rocks dating back to around 3.7 billion years ago, with a peak around 2.5 billion years ago, continuing through the period 2 billion years ago. Their presence indirectly signifies a thriving ecosystem of oxygen-producing life forms.
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Isotopic Signatures: Living organisms preferentially use lighter isotopes of elements like carbon. Analysis of the carbon isotope ratios in ancient rocks provides evidence of biological activity. A significant shift in carbon isotope ratios around 2.7 billion years ago is attributed to the evolution of photosynthesis. Furthermore, rocks from 2 billion years ago display isotopic signatures indicative of continuing biological activity.
In summary, the fossil record, geochemical signatures, and the presence of structures like stromatolites and banded iron formations all point to the existence and activity of life on Earth 2 billion years ago. This life was largely microbial, but it played a crucial role in shaping the Earth's atmosphere and paving the way for the evolution of more complex life forms.
