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Jul 31, 2024 · Dr. Walker’s thinking about life took a major turn in 2015, when she went to a conference in Washington, D.C., on the origin of life. There she listened to Lee Cronin, a chemist at the ...
Instead life almost certainly originated in a series of small steps, each building upon the complexity that evolved previously: 1. Simple organic molecules were formed. Simple organic molecules, similar to the nucleotide shown below, are the building blocks of life and must have been involved in its origin.
- Overview
- Key points:
- Introduction
- When did life appear on Earth?
- The earliest fossil evidence of life
- How might life have arisen?
- From inorganic compounds to building blocks
- Were Miller and Urey's results meaningful?
- From building blocks to polymers
- What was the nature of the earliest life?
The Oparin-Haldane hypothesis, Miller-Urey experiment, and RNA world.
•The Earth formed roughly 4.5 billion years ago, and life probably began between 3.5 and 3.9 billion years ago.
•The Oparin-Haldane hypothesis suggests that life arose gradually from inorganic molecules, with “building blocks” like amino acids forming first and then combining to make complex polymers.
•The Miller-Urey experiment provided the first evidence that organic molecules needed for life could be formed from inorganic components.
•Some scientists support the RNA world hypothesis, which suggests that the first life was self-replicating RNA. Others favor the metabolism-first hypothesis, placing metabolic networks before DNA or RNA.
•Simple organic compounds might have come to early Earth on meteorites.
•The Earth formed roughly 4.5 billion years ago, and life probably began between 3.5 and 3.9 billion years ago.
•The Oparin-Haldane hypothesis suggests that life arose gradually from inorganic molecules, with “building blocks” like amino acids forming first and then combining to make complex polymers.
•The Miller-Urey experiment provided the first evidence that organic molecules needed for life could be formed from inorganic components.
•Some scientists support the RNA world hypothesis, which suggests that the first life was self-replicating RNA. Others favor the metabolism-first hypothesis, placing metabolic networks before DNA or RNA.
If there were other life out there in the universe, how similar do you think it would it be to life on Earth? Would it use DNA as its genetic material, like you and me? Would it even be made up of cells?
We can only speculate about these questions, since we haven't yet found any life forms that hail from off of Earth. But we can think in a more informed way about whether life might exist on other planets (and under what conditions) by considering how life may have arisen right here on our own planet.
Geologists estimate that the Earth formed around 4.5 billion years ago. This estimate comes from measuring the ages of the oldest rocks on Earth, as well the ages of moon rocks and meteorites, by radiometric dating (in which decay of radioactive isotopes is used to calculate the time since a rock’s formation).
For many millions of years, early Earth was pummeled by asteroids and other celestial objects. Temperatures also would have been very high (with water taking the form of a gas, not a liquid). The first life might have emerged during a break in the asteroid bombardment, between 4.4 and 4.0 billion years ago, when it was cool enough for water to condense into oceans1 . However, a second bombardment happened about 3.9 billion years ago. It’s likely after this final go-round that Earth became capable of supporting sustained life.
The earliest evidence of life on Earth comes from fossils discovered in Western Australia that date back to about 3.5 billion years ago. These fossils are of structures known as stromatolites, which are, in many cases, formed by the growth of layer upon layer of single-celled microbes, such as cyanobacteria. (Stromatolites are also made by present-day microbes, not just prehistoric ones.)
The earliest fossils of microbes themselves, rather than just their by-products, preserve the remains of what scientists think are sulfur-metabolizing bacteria. The fossils also come from Australia and date to about 3.4 billion years ago2 .
In the 1920s, Russian scientist Aleksandr Oparin and English scientist J. B. S. Haldane both separately proposed what's now called the Oparin-Haldane hypothesis: that life on Earth could have arisen step-by-step from non-living matter through a process of “gradual chemical evolution.” 3
Oparin and Haldane thought that the early Earth had a reducing atmosphere, meaning an oxygen-poor atmosphere in which molecules tend to donate electrons. Under these conditions, they suggested that:
•Simple inorganic molecules could have reacted (with energy from lightning or the sun) to form building blocks like amino acids and nucleotides, which could have accumulated in the oceans, making a "primordial soup." 3
•The building blocks could have combined in further reactions, forming larger, more complex molecules (polymers) like proteins and nucleic acids, perhaps in pools at the water's edge.
•The polymers could have assembled into units or structures that were capable of sustaining and replicating themselves. Oparin thought these might have been “colonies” of proteins clustered together to carry out metabolism, while Haldane suggested that macromolecules became enclosed in membranes to make cell-like structures4,5 .
The details of this model are probably not quite correct. For instance, geologists now think the early atmosphere was not reducing, and it's unclear whether pools at the edge of the ocean are a likely site for life's first appearance. But the basic idea – a stepwise, spontaneous formation of simple, then more complex, then self-sustaining biological molecules or assemblies – is still at the core of most origins-of-life hypotheses today.
In 1953, Stanley Miller and Harold Urey did an experiment to test Oparin and Haldane’s ideas. They found that organic molecules could be spontaneously produced under reducing conditions thought to resemble those of early Earth.
Miller and Urey built a closed system containing a heated pool of water and a mixture of gases that were thought to be abundant in the atmosphere of early earth (H2 O , NH3 , CH4 , and H2 ). To simulate the lightning that might have provided energy for chemical reactions in Earth’s early atmosphere, Miller and Urey sent sparks of electricity through their experimental system.
Scientists now think that the atmosphere of early Earth was different than in Miller and Urey's setup (that is, not reducing, and not rich in ammonia and methane)6,7 . So, it's doubtful that Miller and Urey did an accurate simulation of conditions on early Earth.
However, a variety of experiments done in the years since have shown that organic building blocks (especially amino acids) can form from inorganic precursors under a fairly wide range of conditions8 .
[What about nucleotides?]
From these experiments, it seems reasonable to imagine that at least some of life's building blocks could have formed abiotically on early Earth. However, exactly how (and under what conditions) remains an open question.
How could monomers (building blocks) like amino acids or nucleotides have assembled into polymers, or actual biological macromolecules, on early Earth? In cells today, polymers are put together by enzymes. But, since the enzymes themselves are polymers, this is kind of a chicken-and-egg problem!
Monomers may have been able to spontaneously form polymers under the conditions found on early Earth. For instance, in the 1950s, biochemist Sidney Fox and his colleagues found that if amino acids were heated in the absence of water, they could link together to form proteins10 . Fox suggested that, on early Earth, ocean water carrying amino acids could have splashed onto a hot surface like a lava flow, boiling away the water and leaving behind a protein.
Additional experiments in the 1990s showed that RNA nucleotides can be linked together when they are exposed to a clay surface11 . The clay acts as a catalyst to form an RNA polymer. More broadly, clay and other mineral surfaces may have played a key role in the formation of polymers, acting as supports or catalysts. Polymers floating in solution might have hydrolyzed (broken down) quickly, supporting a surface-attached model12 .
The image above shows a sample of a type of clay known as montmorillonite. Montmorillonite in particular has catalytic and organizing properties that may have been important in the origins of life, such as the ability to catalyze formation of RNA polymers (and also the assembly of cell-like lipid vesicles)13 .
If we imagine that polymers were able to form on early Earth, this still leaves us with the question of how the polymers would have become self-replicating or self-perpetuating, meeting the most basic criteria for life. This is an area in which there are many ideas, but little certainty about the correct answer.
Sep 19, 2022 · Earth is about 4.5 billion years old. Scientists think that by 4.3 billion years ago, Earth may have developed conditions suitable to support life. The oldest known fossils, however, are only 3.7 billion years old. During that 600 million-year window, life may have emerged repeatedly, only to be snuffed out by catastrophic collisions with ...
The history of life on Earth traces the processes by which living and extinct organisms evolved, from the earliest emergence of life to the present day. Earth formed about 4.5 billion years ago (abbreviated as Ga, for gigaannum) and evidence suggests that life emerged prior to 3.7 Ga. [1] [2] [3] The similarities among all known present-day ...
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Sep 1, 2009 · The earliest forms of life could have been simple membranes made of fatty acids—also structures known to form spontaneously—that enveloped water and these self-replicating genetic molecules ...
