Life on Earth began more than 3 billion years ago, developing over time from the simplest of microorganisms to an astounding diversity of complexity. However, how did the first species on the universe's only known home for life evolve from the primordial soup?
As demonstrated in the famous Miller-Urey experiment published in 1953, electric sparks can generate amino acids and sugars from an atmosphere rich in water, methane, ammonia, and hydrogen, implying that lightning may have aided in the formation of the critical building blocks of life on Earth in its early days.
Larger and more complex molecules may emerge over millions of years. Although the subsequent study has proved that the Earth's early atmosphere was genuinely devoid of hydrogen, scientists have speculated that volcanic clouds in the early atmosphere may have contained methane, ammonia, and hydrogen, as well as being filled with lightning.
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According to a concept advanced by organic chemist Alexander Graham Cairns-Smith of the University of Glasgow in Scotland, the initial molecules of life may have collided on clay. These surfaces may have not only condensed these chemical substances but also organized them into patterns similar to how our genes do now.
DNA's primary function is to hold instructions on how other molecules should be organized. DNA sequences are essentially instructions for the arrangement of amino acids in proteins. Cairns-Smith hypothesizes that mineral crystals in clay may have structured organic molecules into patterns. Organic molecules eventually took over and arranged themselves.
According to the deep-sea vent theory, life may have originated at submerged hydrothermal vents that emitted critical hydrogen-rich compounds. Their rocky nooks may have then concentrated these chemicals and acted as mineral catalysts for crucial processes. Even today, these vents, which are a source of chemical and thermal energy, support thriving organisms.
Ice may have blanketed the oceans 3 billion years ago when the sun was approximately a third as bright as it is now. This layer of ice, which may have been hundreds of feet thick, may have shielded the light organic chemicals in the water below from UV light and cosmic impact damage. In addition, the cold may have aided in the survival of these molecules, allowing critical processes to occur.
Nowadays, DNA requires proteins to form and DNA to develop, so how could these two molecules have formed independently? The answer may be RNA, which, like DNA, can store information, act as an enzyme, and aid in creating both DNA and proteins.
Later, DNA and proteins surpassed this "RNA universe" inefficiency. Nevertheless, RNA is still present in organisms and serves a variety of roles, including acting as an on-off switch for some genes. The question of how RNA got here remains unanswered.
And, while some scientists believe the molecule may have formed spontaneously on Earth, others believe this is highly improbable. Other nucleic acids, such as the more esoteric PNA or TNA, have been suggested as well.
Instead of beginning with sophisticated molecules such as RNA, life may have started with smaller molecules interacting in cycles of reactions. These may have been contained in straightforward capsules resembling cell membranes, and over time, more complex molecules capable of performing these reactions better than the simpler ones may have evolved, a scenario dubbed "metabolism-first" in contrast to the "gene-first" model of the "RNA world" hypothesis.
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Perhaps life did not originate on Earth but was delivered here via panspermia from elsewhere in space. For example, rocks are periodically blasted off Mars by cosmic impacts, and several Martian meteorites have been discovered on Earth, with some experts speculating that they took bacteria with them, potentially transforming us all into Martians.
Others have speculated that life may have arrived on comets from other star systems. However, even if this hypothesis were true, the topic of how life originated on Earth would shift to how life originated elsewhere in space.
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