Unit 1: Many Planets, One Earth // Section 7: Early Life: Single-Celled Organisms
We do not know exactly when life first appeared on Earth. There is clear evidence that life existed at least 3 billion years ago, and the oldest sediments that have been discovered—rocks formed up to 3.8 billion years ago—bear marks that some scientists believe could have been left by primitive microorganisms. If this is true it tells us that life originated soon after the early period of bombardment by meteorites, on an Earth that was probably much warmer than today and had only traces of oxygen in its atmosphere.
For the first billion years or so, life on Earth consisted of bacteria and archaea, microscopic organisms that represent two of the three genealogical branches on the Tree of Life (Fig. 15). Both groups are prokaryotes (single-celled organisms without nuclei). Archaea were recognized as a unique domain of life in the 1970s, based on some distinctive chemical and genetic features. The order in which branches radiate from the Tree of Life shows the sequence in which organisms evolved. Reading from the bottom up in the same way in which a tree grows and branches outward, we can see that eucarya (multi-celled animals) were the last major group to diverge and that animals are among the newest subgroups within this domain.
Figure 15. The universal Tree of Life
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Source: © Jack D. Farmer, 2000. Hydrothermal Systems: Doorways to Early Biosphere Evolution, GSA Today 10(7), 1-9.
Life on Earth existed for many millions of years without atmospheric oxygen. The lowest groups on the Tree of Life, including thermatogales and nearly all of the archaea, are anaerobic organisms that cannot tolerate oxygen. Instead they use hydrogen, sulfur, or other chemicals to harvest energy through chemical reactions. These reactions are key elements of many chemical cycles on Earth, including the carbon, sulfur, and nitrogen cycles. "Prokaryotic metabolisms form the fundamental ecological circuitry of life," writes paleontologist Andrew Knoll. "Bacteria, not mammals, underpin the efficient and long-term functioning of the biosphere" (footnote 5).
Some bacteria and archaea are extremophiles—organisms that thrive in highly saline, acidic, or alkaline conditions or other extreme environments, such as the hot water around hydrothermal vents in the ocean floor. Early life forms' tolerances and anaerobic metabolisms indicate that they evolved in very different conditions from today's environment.
Microorganisms are still part of Earth's chemical cycles, but most of the energy that flows through our biosphere today comes from photosynthetic plants that use light to produce organic material. When did photosynthesis begin? Archaean rocks from western Australia that have been dated at 3.5 billion years old contain organic material and fossils of early cyanobacteria, the first photosynthetic bacteria (footnote 6). These simple organisms jump-started the oxygen revolution by producing the first traces of free oxygen through photosynthesis: Knoll calls them "the working-class heroes of the Precambrian Earth" (footnote 7).
Cyanobacteria are widely found in tidal flats, where the organic carbon that they produced was buried, increasing atmospheric oxygen concentrations. Mats of cyanobacteria and other microbes trapped and bound sediments, forming wavy structures called stromatolites (layered rocks) that mark the presence of microbial colonies (Fig. 16).
Figure 16. Stromatolites at Hamelin Pool, Shark Bay, Australia
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Source: © National Aeronautics and Space Administration, JSC Astrobiology Institute.
The third domain of life, eukaryotes, are organisms with one or more complex cells. A eukaryotic cell contains a nucleus surrounded by a membrane that holds the cell's genetic material. Eukaryotic cells also contain organelles—sub-components that carry out specialized functions such as assembling proteins or digesting food. In plant and eukaryotic algae cells, chloroplasts carry out photosynthesis. These organelles developed through a process called endosymbiosis in which cyanobacteria took up residence inside host cells and carried out photosynthesis there. Mitochondria, the organelles that conduct cellular respiration (converting energy into usable forms) in eukaryotic cells, are also descended from cyanobacteria.
The first eukaryotic cells evolved sometime between 1.7 and 2.5 billion years ago, perhaps coincident with the rise in atmospheric oxygen around 2.3 billion years ago. As the atmosphere and the oceans became increasingly oxygenated, organisms that used oxygen spread and eventually came to dominate Earth's biosphere. Chemosynthetic organisms remained common but retreated into sediments, swamps, and other anaerobic environments.
Throughout the Proterozoic era, from about 2.3 billion years ago until around 575 million years ago, life on Earth was mostly single-celled and small. Earth's biota consisted of bacteria, archaea, and eukaryotic algae. Food webs began to develop, with amoebas feeding on bacteria and algae. Earth's land surfaces remained harsh and largely barren because the planet had not yet developed a protective ozone layer (this screen formed later as free oxygen increased in the atmosphere), so it was bombarded by intense ultraviolet radiation. However, even shallow ocean waters shielded microorganisms from damaging solar rays, so most life at this time was aquatic.
As discussed in sections 5 and 6, global glaciations occurred around 2.3 billion years ago and again around 600 million years ago. Many scientists have sought to determine whether there is a connection between these episodes and the emergence of new life forms around the same times. For example, one Snowball Earth episode about 635 million years ago is closely associated with the emergence of multicellularity in microscopic animals (footnote 8). However, no causal relationship has been proved.