SC.912.L.15.8: Describe the scientific explanations of the origin of life on Earth.
Where Did Life Come From?
The origins of life on Earth are a matter of scientific debate. The prevailing theory is that life emerged independently from different parts of the world. The first forms of life were single-celled microorganisms that lived in the oceans. Over time, these microorganisms evolved into more complex forms, eventually leading to the emergence of all the biodiversity that we see today.
Environment of Early Earth
Once the Earth was formed it was likely an inhospitable inferno, definitely not an environment suitable for life as we know it today. After about 500 million years the Earth was finally cool enough to allow liquid water to form on the surface. Oceans, seas, rivers, and lakes eventually formed. Since photosynthetic organisms had not yet appeared it is believed that the Atmosphere of early Earth lacked Oxygen gas (O2) and so the composition of the atmosphere may have been very different. Scientists believe the atmosphere of ancient Earth consisted of primarily; H2O (water), CH4 (methane), NH3 (ammonia), and H2 (hydrogen gas). According to the fossil record as soon as the conditions were right, microbial life formed.
Chemical Evolution
Chemical evolution is the theory that the life and its organic compounds (carbon containing chemicals) developed from inorganic chemicals present on early Earth. This theory proposes that simple inorganic chemicals if given enough time and provided enough energy can react to form more complex molecules that are necessary for life, and that these complex “carbon based” chemicals can then form the first organisms. There is actually a significant amount of information and scientific literature that supports this theory in some form or another, one such example if the famous experiment conducted by Stanley Miller and supervised by Harold Urey.
Miller-Urey Experiments
At Chicago University in 1952 a then graduate student Stanley Miller designed and conducted an experiment to test if the pre-biotic conditions of Earth were sufficient to generate life or the chemicals associated with it. Miller constructed a glass apparatus that would simulate the processes likely present on prebiotic Earth. He created a sealed system in which he boiled water and forced it to recirculate, condense, and form liquid water to simulate rain and the water cycle. He removed all air from the system and pumped in the gasses that scientist believed the early atmosphere was rich in. He also included electrodes which were used to simulate lightning bolts and provide a source for the energy needed for chemical reactions to occur. He many other scientists assumed these conditions could create a “Primordial Soup“, a pool of liquid rich with all the things needed for life to be generated.
After running his experiment for a number of days he analyzed what chemicals were present in the water. He did not find cellular life, but interestingly he was able to detect the presence of amino acids and other carbon molecules. This was a ground breaking discovery at the time and laid the foundation for many similar experiments in the future. Scientists have since tweaked and repeated Miller’s experiment and found similar results, some were even able to produce nucleic acids (the monomers that encode genetic information). Conditions similar to those Miller proposed are present throughout our galaxy and other regions of space. It is possible that the chemicals necessary for life seeded the Earth through a process known as “Panspermia“.
Geohydrothermal Vent Theory
Hydrothermal vents are large chimney like structures that are typically formed on the ocean floors. These vents typically form where continental plates meet underwater. Similar to hotsprongs and geysers we would find above ground the vents release massive amounts of super heated water with temperatures up to 700° F. Interestingly the vents also release incredibly toxic chemicals like; hydrogen sulfide, hydrogen gas, ferrous iron and ammonia.
You would assume that nothing would survive around the vents but you would be wrong. Millions of tiny bacteria feed off the chemicals produced by the vents in a process called “Chemosynthesis”, they use the inorganic chemicals from the vent to produce glucose. The bacteria’s ability to survive such extremely harsh conditions has made scientist classify them as “extremophiles“, organisms that are capable of surviving conditions we would not expect life to be possible in. For a more detailed discussion of extremophiles check out our article here.
Geohydrothermal Vent Theory proposes that the intense heat from the vents provided enough energy for the inorganic molecules present to react and form organic compounds needed for life. That these organic compounds with enough time eventually formed the first organisms. Scientist have found fossils of the first bacterial life forms, these fossils are called “stromatolites“. The modern day extremophile bacteria that surround the hydrothermal vents are very similar to these fossilized bacteria.
Panspermia
Panspermia is the idea that life or the chemical components necessary for life originated in space. Interestingly there is a significant evidence that supports this theory of how life emerged.
Panspermia often invokes mental images of little grey men and flying saucers, but there is very little evidence for that scenario.
Meteorites have been found to contain various macromolecules such as; sugars, amino acids, and even nucleic acids. Panspermia may explain how life appeared so quickly in the fossil record right after the Earth cooled. It is possible that a meteorite carrying microbial life found a barren but hospitable Earth with liquid water. For a more detailed discussion on the topic of “Panspermia” please read our article here.
There are a number of theories about how life first emerged on Earth. Regardless of how it started, it is clear that life on Earth has evolved over billions of years into the rich and varied ecosystems we see today.