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8.1: Evidence for the Emergence of Life

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  • Where did life arise on Earth?

    It is unclear whether the transition from chemistry to biology occurred on the land surface of Earth or in the oceans. Those intermediate species on our planets - entities that are more than chemistry, but less than life - no longer seem to exist.

    In the mid-1960s, Alexander Graham Cairns-Smith proposed that complex prebiotic molecules may have first organized around the regular crystal patterns found in minerals in clay. Cairns-Smith hypothesized that electrostatic forces in mineral crystals would help to concentrate and align specific molecules to their surface. With the help of this clay, precursors of RNA could have assembled, triggering the start of an RNA world. Tide pools have also been proposed as the site where chemistry developed the complexity needed for life. The cycles of evaporation and subsequent addition of water in tide pools might have concentrated the organic materials necessary to form life. Higher concentrations increase the likelihood that these organic materials will react and form RNA or lipids.

    Far from the rocky surface of Earth, it is possible that life originated deep in the oceans when our planet was a frozen world. During the phases where Earth was covered by a thick mantle of ice, the ice may have protected organic compounds beneath the ice from the damage of impacting asteroids or ultraviolet radiation from the sun.

    Deep-sea, hydrothermal vents are hot spots that release gas from the Earth's interior. DNA sequencing suggests that LUCA was a thermophilic microbe and deep sea vents might have been ideal locations for nurturing that early life. At first glance, this seems an inhospitable place for life. These vents are located in some of the darkest, highest pressure environments on our world. However, the gas that is released from hydrothermal vents creates a natural chemical gradient that can be used by living cells to generate the energy needed for metabolism. In deep hydrothermal vents, this chemical gradient flows around the rocks surrounding the vent and creates nooks where organic compounds can collect and reach higher concentrations. Minerals that are capable of acting as catalysts have been found in deep sea vents. Therefore, hydrothermal vents seem to provide the needed ingredients: high concentrations of organic compounds, natural catalysts, and a powerful energy source in the absence of sunlight. Today, extremely varied ecosystems are found around these vents.

    Figure \(\PageIndex{1}\): A deep-sea hydrothermal vent. These vents release a cocktail of varied chemicals and ions that contain the starting materials for life and create a helpful chemical gradient that can be used as a source of natural energy.

    When did life arise on Earth?

    The evidence for timing the initial rise of life on our planet is ambiguous. The most ancient evidence has been destroyed as the first forms of life would have been single-celled organisms that did not leave behind fossil remains. However, suggestive evidence from biology, chemistry, and geology supports estimates that life arose on Earth between 3 to 4 billion years ago. Since the Earth formed about 4.56 billion years ago, the early appearance of life hints that the evolution from chemistry to life may be statistically probable. If so, this greatly increases the odds that life has also evolved elsewhere.

    1. Carbon isotopes

    The earliest line of evidence for timing the rise of life comes from carbon isotopes. Carbon naturally occurs in three isotopes, always with 6 protons, but with either 6, 7, or 8 neutrons, which are annotated 12C, 13C, and 14C respectively. The superscripts indicate the atomic mass number, or the total number of protons and neutrons in an element. Of these isotopes, 14C will undergo radioactive decay with a half life of 5730 years. However, 12C and 13C do not decay and therefore, the ratio of 12C to 13C is constant over time on the Earth. However, life preferentially incorporates the lighter 12C, rejecting 13C. Therefore, a low ratio of 13C to 12C offers a tentative suggestion of organic material existed and that isotopic imbalance can be incorporated into the metamorphic structure of the rock.

    Why does life prefer to use 12C? While the chemical reactivity of an atom is predominantly driven by the electron configuration, more massive isotopes tend to have slightly slower reaction rates. That slight edge in the speed of reaction rates is enough of an advantage to favor the uptake of carbon-12 over carbon-13 in organic biochemistry.

    Zircon crystals are commonly used to assess ages in very ancient geological records because they are very durable minerals, resistant to both heat and corrosion. Trapped minerals can be preserved when zircons form. A group of scientists from the University of California, Los Angeles studied 10,000 zircons gathered from western Australia. Of these ten thousand zircons, one contained a graphite inclusion, a compound composed entirely of carbon atoms.

    Figure \(\PageIndex{2}\): A hearty zircon crystal. Zircon crystals are not only capable of surviving through great stressors, but they are also good at keeping elements trapped inside them. Why are both of these properties important for accurate radiometric dating?

    Radiometric dating showed that this zircon crystal was 4.1 Gyr old and the carbon inclusion exhibited a larger ratio of 12C to 13C. Was this the chemical stain of primordial life, dating back to 4.1 Gya? On Earth today, this would be a good indicator of organic material. But, interpreting this result from so long ago is more controversial.

    2. Stromatolites

    Ancient stromatolites offer a more secure timeline for the emergence of life. These fossilized structures are found in shallow waters and look like modern structures formed by cyanobacteria. The oldest stromatolites have been found in South Africa and Australia and date back to the early Archean Period between 3.2 - 3.5 Gya. By the end of the Archean and throughout the Proterozoic geological periods, stromatolites appear to have been abundant and formed the first reefs.

    Figure \(\PageIndex{3}\): Fossilized stromatolites. This distinctive banding pattern resembles structures produced by living organisms today. This suggests that similar organisms existed at the time these fossils were formed.

    Stromatolites today are composed of layers of microbial mats of photosynthesizing cyanobacteria, as shown in the YouTube video below. Photosynthesis depletes carbon dioxide in the water, precipitating calcium carbonate deposits, which along with other sandy sediments, is trapped in the sticky bacterial film. The bacterial colonies grow upward towards better exposure to the sun and over time, the layers of bacterial film and mineral precipitates build up to form distinctive layered stromatolites.Similar stromatolite structures are formed today.

    We guess that ancient stromatolites were formed through biological processes, since we observe this behavior in microbes today. The processes required to make these structures seems to require microbes that are already quite complex and capable of photosynthesis. Actual first life then, likely to be a more primitive organism, would have existed even earlier than these ~3.5 Gyr old structures.

    3. Microfossils

    The most direct evidence for life comes in the form of microfossils, fossils preserving micro-organisms that may have been among the first living creatures on Earth. The most convincing microfossils date back to approximately 3 billion years ago. However, these are difficult to identify both because rocks undergo erosion over time and because the structures that resemble microfossils might actually be formed by nonbiological processes. Many supposed microfossils turn out to be false positives after careful chemical analysis. Though the discovery of a microfossil is an unmistakable mark of life, it is easy to be fooled. The figure below gives you a good idea of the scant information content in microfossils.

    Figure \(\PageIndex{4}\): An example of possible microfossils. A micrometer is a millionth of a meter. Human hair is on average 80 micrometers thick.

    4. Great Oxidation Event

    The Great Oxidation Event is alternatively called the Oxygen Catastrophe depending on whether you ask aerobes or anaerobes. It marks a drastic change in the Earth's atmospheric composition that occurred about 2.5 Gya.

    An increase in atmospheric oxygen is seen in the geological record as a sudden onset of oxidation of iron occurred. Iron oxidation coincides with the geological evidence for glaciation 2.5 billion years ago. Oxygen is not a greenhouse gas, but it is a highly reactive species that would have interacted with the primitive methane atmosphere produced by volcanic outgassing. Methane is a powerful greenhouse gas. As oxygen levels rise, oxygen would react with methane to form carbon dioxide and water. This mechanism for removing methane would have produced the significant cooling and widespread glaciation. An outcrop of ancient Canadian rocks shows evidence of a glaciation event between different rock layers. Below (and therefore older than) this glaciation event, the rocks are consistent with low levels of atmospheric oxygen. Younger rock above the glaciation layer show significantly higher oxidation. Radiometric dating of the rock show that the increase in atmospheric oxygen, or the Great Oxidation Event, occurred 2.45 billion years ago.

    The addition of oxygen to the atmosphere is believed to be the result of photosynthesis from cyanobacteria. Oxygen did not accumulate immediately because of the enormous number of sinks, including reduced gases and minerals that would have overwhelmed its production. Those sinks for oxygen are now largely saturated; respiration of anaerobic organisms (including us) and decay of organic material take up most of the oxygen produced today.

    As the atmosphere cooled, a positive feedback loop ensued. Water vapor, which is another important greenhouse gas would have condensed out of the atmosphere. The Earth eventually froze over and became what is known as ''snowball Earth." Volcanic activity would have continued during this phase, releasing internal heat, along with carbon dioxide into the air. With a sufficient amount of CO2, the greenhouse warming would have melted the ice, allowing the Earth to recover from its frozen state. Assuming present rates of volcanism, the necessary build up of CO2 would take 10 million years. Evidence in the rock record suggests that one snowball Earth event occurred around 2.2 billion years ago.

    Figure \(\PageIndex{5}\): An artist rendition of how the Snowball Earth may have appeared. The extent of glaciation would have been worldwide, causing a thick layer of ice from pole to pole.