London, May 20 (Dialogue) From its humble beginnings, life has infected the entire planet in its endlessly beautiful forms. The origin of life is the oldest biological event, so old that it leaves no definitive evidence for anything other than the existence of life itself.
This leaves many unanswered questions, one of the most tantalizing being how many times life has magically emerged from non-living elements.
Did all life on Earth evolve only once, or were different creatures cut from different fabrics? The question of how hard it is for life to emerge is interesting — not least because it could reveal the possibility of finding life on other planets.
The origin of life is a central question in modern biology, and perhaps the most difficult to study. The event happened 4 billion years ago, and at the molecular level — meaning there’s little fossil evidence.
Many vivid beginnings have been proposed, from geeky primordial soup to outer space. But the current scientific consensus is that life arose from nonliving molecules in a natural process called abiogenesis, likely in the darkness of deep-sea hydrothermal vents. But if life occurs once, why not more times?
What is naturally occurring?
Scientists have proposed various sequential steps that occur naturally. We know that Earth is rich in a variety of chemicals, such as amino acids, molecules called nucleotides or sugars, which are the building blocks of life. Laboratory experiments, such as the iconic Miller-Urey experiment, have shown how these compounds formed naturally under conditions similar to early Earth. Some of these compounds may also have come to Earth on meteorites.
Next, these simple molecules combine to form more complex molecules such as fats, proteins or nucleic acids. Importantly, nucleic acids — such as double-stranded DNA or its single-stranded cousin RNA — can store the information needed to build other molecules. DNA is more stable than RNA, but RNA, by contrast, can participate in chemical reactions in which a compound replicates itself—self-replicating.
The “RNA world” hypothesis suggests that early life may have used RNA as the material for genes and replication, before the advent of DNA and proteins.
Once an information system can replicate itself, natural selection begins. Some of the new copies of these molecules (what some call “genes”) will have errors or mutations, and some of these new mutations will increase the ability of the molecule to replicate.
So over time, these mutants will have more copies than others, and some of them will accumulate more new mutations, making them faster, more abundant, and so on.
Eventually, these molecules may have evolved a lipid (fat) boundary that separated the organism’s internal environment from its external environment, forming the protocell. Protocells are better able to concentrate and organize the molecules needed for biochemical reactions, providing a closed and efficient metabolism.
Spontaneous occurrence may occur more than once. Earth may have produced self-replicating molecules many times, and perhaps thousands or millions of years of early life simply consisted of a bunch of different self-replicating RNA molecules, with independent origins, competing for the same building blocks. Alas, due to the ancient and microscopic nature of this process, we may never know.
Many laboratory experiments have successfully reproduced different stages of natural occurrence, proving that they may have occurred more than once, but we are not sure whether these occurred in the past.
A related question might be whether new life is emerging through aeogenesis as you read this. But that’s unlikely. The early Earth was lifeless and had very different physical and chemical conditions. Today, if somewhere on Earth there are ideal conditions for the emergence of new self-replicating molecules, they are quickly consumed by existing life.
What we do know is that all extant life arose from a common Last Universal Common Ancestor of Life (also known as LUCA). If there are other ancestors, they don’t leave any descendants. Key evidence supports the existence of LUCA.
All life on Earth uses the same genetic code, the correspondence between nucleotides called A, T, C and G in DNA and the amino acids they encode in proteins. For example, the sequence of three nucleotides ATG always corresponds to the amino acid methionine.
In theory, however, there could be more variation in the genetic code between species. But all life on Earth uses the same code, with slight variations in certain lineages. Biochemical pathways, such as those used to metabolize food, also support the existence of LUCA; many independent pathways may have evolved in different ancestors, but some, such as those used to metabolize sugars, are common in all organisms shared. Also, hundreds of identical genes exist in different organisms, which can only be explained by inheritance from LUCA.
My favorite support for LUCA is the tree of life. Several independent analyzes using anatomical, metabolic or genetic sequences have revealed a hierarchical pattern of relatedness that can be represented as a tree. This suggests that we are more closely related to chimpanzees than to any other creature on Earth. Chimpanzees and we are more closely related to gorillas, more closely related to orangutans, and so on.
You can pick any organism at will, from lettuce in salads to bacteria in bioactive yogurt, and if you go back far enough in time, you’ll have a true common ancestor. This is not a metaphor, but a scientific fact.
This is one of the most incredible concepts in science, Darwin’s unity of life. If you’re reading this, you’re here thanks to an unbroken chain of reproductive events billions of years ago. It’s exciting to think about life recurring on our planet or elsewhere, but even more exciting is knowing that we are related to all life on Earth. (dialogue)
(This is an unedited and auto-generated story from a Syndicated News feed, the content body may not have been modified or edited by LatestLY staff)