Dawn of Animal Life
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Approximately 1.5 billion years ago Eukaryotic Cells, the next evolutionary step.
Differences between plants and animals seem at first to mark a basic division of life on Earth. Scientists however recognize an even more fundamental division: life based on prokaryotic cells versus life based on eukaryotic cells. Bacteria and cyanobacteria are primitive prokaryotic life forms, while all other plants and animals are based on more complex eukaryotic cells.
One of the earliest forms of life was probably single-celled bacteria based on the prokaryotic cell (like cyanobacteria in stromatolites). This is the simpler of the two types of carbon-based cells known today, and it was the first one to evolve.
In the prokaryotic cell, a cell wall encloses a liquid called cytoplasm within which is a single strand of DNA attached at one point to the plasma membrane. There is very little internal organization within the cell. Prokaryotes multiply by simple cell division, forming clones or exact copies of themselves. Since the offspring shares the exact same genetic code as the parent, their evolution is very slow.
Eukaryotic cells are much larger and more advanced. Inside the cell, genetic material (DNA) is packaged within a nucleus, and discrete organelles perform specific jobs such as respiration and photosynthesis. These cells need oxygen to survive and they most likely evolved and lived near oxygen-producing stromatolites at first. Most importantly, eukaryotes reproduce sexually. Genetic material is mixed between two parents, and while the offspring resembles the parents in major features, minor genetic variations can yield subtle differences. The new characteristics sometimes help eukaryotic organisms to adapt to new environments.
Fossils Made of Carbon
Life on Earth is based on the carbon atom, which is bonded to oxygen, hydrogen, nitrogen and other elements. Often, when life-forms are buried and fossilized, the volatile elements (including hydrogen, oxygen, and nitrogen) are driven off and the only element that survives the process is carbon. In such cases, the overall shape of an organism can be seen in the rocks as a carbon layer, or carbonaceous compression.
Carbonaceous fossils are found in rocks up to 2 billion years old. They represent a variety of organisms including colonies of prokaryotes, single-celled eukaryotes, and fossil seaweeds. In Canada, the carbon fossils Tawuia and Chuaria are found; they may have been large eukaryotic cells.
Chuaria circularis (top image at left) and Tawuia dalensis (bottom image at left) are found in approximately 800 million year old rocks exposed west of Norman Wells, NWT. These disk (Chuaria) and ribbon-like (Tawuia)carbonaceous fossils are found associated with stromatolitic reefs, exactly the setting envisioned for early eukaryotic cells.
Because they are often found together, some scientists have suggested that Chuaria and Tawuia represent different life stages of the same organism, possibly a eukaryotic alga. One may have been the free-floating or planktonic stage, and the other a cystlike resting stage.
The next stage in evolution of animal life was the development of metazoans, the multi-celled animals.