It's easy to take the life that our planet teems with for granted, but its existence raises a very difficult question: Where did life on Earth come from?
Prior to the 1860s, when Louis Pasteur developed his germ theory of disease, the common consensus was that life could spontaneously generate. For example, if you left wheat grain out, it would spawn baby mice. Water left standing for days would birth organisms like algae. Today we know new life won't generate out of nothing, but this is due largely to Pasteur's work [source: Abedon].
Pasteur showed us that microorganisms, incapable of detection by human senses, live everywhere around us. This led to his germ theory, which states that human health is subject to attack by these microorganisms and that these attacks can result in what we think of as disease.
But his legacy also includes a sea change in human understanding about the genesis of life. After Pasteur revealed the nature of germs, he effectively killed the now-seemingly superstitious idea that organic life can generate spontaneously out of inorganic material. Ironically, science has come back to explore this very concept as a possibility once more as one of two main competing explanations for the origin of life on Earth.
The concept that life may have spontaneously generated is called abiogenesis. In the remote past precursors to life like amino acids and proteins arose from a primordial soup and managed to arrange themselves into self-replication precellular life forms. This beginning of life eventually composed and transcribed the DNA that forms the basis of the genetic code of life processes today. It's a fantastic idea -- and one that many inside and outside of the scientific community criticize.
In the other corner is abiogenesis' main -- and equally fantastic -- rival as explanation for the origin of life on Earth. This concept, panspermia, says that life didn't begin here on Earth, but elsewhere in the universe or solar system. Life was carried here, in a vehicle like an asteroid from another planet, and took hold in much the same way that a seed does in fertile soil. Probably more accurately, life would've spread like an epidemic disease in a form very similar to the germs that Pasteur uncovered.
No one can be sure which one adequately explains the origin of life on Earth, but amazingly, both have been shown to be possible. In this article, we'll look at the case each makes. First, we'll look at a common problem that both theories share.
Darwin and Phylogeny
At around the same time that Pasteur developed his germ theory, Charles Darwin was introducing his theory of evolution to the world. It would contribute what constitutes a logical road map in the search for the first life on Earth. In "The Origin of Species," Darwin references Sir John Herschell's description of the genesis of life on earth as the "mystery of mysteries" and proposes that the species on Earth today weren't independently created. Instead, they evolved in ever-expanding numbers from earlier species through the process of evolution by natural selection [source: Darwin]. At the heart of this line of reasoning is the implication that all organisms could have evolved from a single common ancestor. Thus, the modern investigation into the origin of life on Earth began.
Darwin's work was built upon an already existing system of biological classification proposed in 1753 by Swedish biologist Carl von Linne (known as Linnaeus). Linnaeus developed taxonomy, a system for classifying organisms based generally on physical traits, from the narrowest taxon (species) to a group of related species (genus) and into increasingly broader taxa up to the kingdoms of plants and animals (and originally minerals) [source: Pidwirny]. This system of biological classification has itself evolved over time, with the number of kingdoms expanding and the broadest taxon, domains, established to categorize cells as eukaryotic (containing cells with DNA in a nucleus), bacteria and archaea (the domain of extremophiles).
Over time taxonomy has become more accurate, through the application of genetics. This hybrid field is called phylogeny, where the interrelatedness of organisms is established based on their shared DNA. For example, the related genes (those performing similar functions) found in humans and some types of mice share as much as 90 percent similarity in their DNA sequences [source: Stubbs]. Genetic comparison of chimps and humans yield about 95 percent similarity [source: Pickrell]. These similarities are significant but phylogeny has confirmed what Linnaeus, Darwin and countless other scientists have long postulated -- that every living thing on Earth is related.
The system used to classify living things looks a lot like a tree, with the early organisms making up the root structure, and various taxa narrowing into the trunk, large branches, smaller branches and finally into the leaves that represent the nearly 2 million species currently classified by science [source: O'Loughlin]. This representation is often called the tree of life. Yet as phylogeny has come increasingly into use, it's shown that perhaps the roots of the tree of life are somewhat atypical.
The Trouble with the Tree of Life
The genetic comparison of organisms provided by phylogeny has revealed a serious obstacle to tracing the tree of life back to the single common ancestor that earlier biologists couldn't see. The hunt for the common ancestor -- and the very idea that one existed -- is based on genetic distribution via vertical gene transfer. Through this, genes are passed along from one generation to the next through sexual or asexual reproduction. Either one or two organisms give rise to another that inherits a replica of itself or a predictable combination of their genes. Over time, organisms may eventually diverge into entirely different species or even kingdoms, like humans from apes (or, even further back, where the lineage that gave rise to birds diverged from that of bacteria), but this horizontal transfer of genes still leaves a trail of genetic bread crumbs we can follow to trace our origins.
That genes were only transferred vertically was the predominant view of scientists until the 1950s when another type of gene transfer was discovered. Horizontal or lateral gene transfer is another means of one organism obtaining another's genes, but rather than parent to offspring, this method of genetic distribution is based on one organism effectively absorbing another organism's DNA whole and intact [source: Wade]. Two organisms can create a third, seemingly unrelated hybrid organism with both genes, but not in any way similar to the equal combining of genes that occurs during reproduction. Instead, one larger organism can virtually eat another organism and retain the second organism's genetic code, using the first organism's code for itself. The mitochondria, the part of the cell responsible for converting sugars to the energy used to power cellular functions in eukaryotic animals, is thought to have once existed as an independent organism [source: Wade]. Through lateral transfer, an ancient eukaryote absorbed it and retained its genetic make-up.
Early in Earth's history, microbiologists now believe lateral transfer was common, giving the roots of the tree of life not a direct line upward from a single seed, but rather a series of impossibly criss-crossed, virtually untraceable lines among single-celled organisms. The search for a single common ancestor was dealt another blow after research showed extremophiles, organisms capable of surviving in harsh conditions and candidates for the earliest life forms on Earth, likely evolved from other bacteria and later adapted to their environments [source: Zimmer]. This suggests they are less ancient than previously thought.
But whether we evolved from a single common ancestor or many, the question remains, how did life on Earth begin? We get closer to the answer on the next page.
Abiogenesis and the RNA World
Here we arrive back at the beginning, as it were. In the 1950s, a graduate student at the University of Chicago named Stanley Miller sought to recreate the conditions found on Earth approximately 3.8 billion years ago, around the time the fossil record first showed life [source: Zimmer]. Miller designed an ingenious and now famous experiment where he added approximate measurements of hydrogen, methane and ammonia into a flask containing water. This element and compounds were thought predominant in the atmosphere of the young Earth. When Miller simulated lightning by adding a spark he found that the solution in his flask now contained something it hadn't before: amino acids.
Amino acids are commonly called the building blocks of life, as they provide the foundation for proteins, which are necessary for organisms' structure and functions. Miller's experiments have held up. For example, an experiment that included hydrogen sulfide and a jet of steam, which simulates the presence of volcanic activity, was later found to be a fairly precise approximation of the early Earth from research that came after Miller's death [source: NASA]. Another implicated formaldehyde as a catalyst for the origin of life [source: Science Daily]. These experiments yielded even more convincing evidence that life on Earth arose from abiogenesis.
The foundation of abiogenesis is that pre-cellular life once existed on Earth. These precursors to life assembled themselves from the amino acids present in the primordial soup recreated by Miller and became the proteins that provide structure to cells and act as enzymes for cellular processes. At some point, these proteins formed genetic templates so that they could be replicated and organized themselves into organelles like ribosomes, which transcribe molecules from these templates [source: Science Daily]. Eventually, these processes came together to create DNA, which forms the basis of cellular life.
Abiogenesis as a theory for the origin of life got a boost in the 1980s when researcher Thomas Cech proved that RNA can act as both a bearer of genetic code as well as an enzyme that catalyzes that code into the creation of molecules. This finding gave rise to the RNA world hypothesis, which is the idea that amino acids first formed into the proteins that make up ribonucleic acid (RNA), which took over and began self-replicating and generating new combinations of proteins creating new pre-cellular -- and eventually cellular -- life.
Under abiogenesis, organic life was created randomly from the inorganic components of life. Its scientific competitor envisions a different beginning to life on Earth.
Panspermia: Life from Outer Space
The principle behind panspermia is that life originated outside of Earth and traveled to our planet, finding a hospitable climate in which to thrive and eventually evolve into life on Earth.
Panspermia is an old concept, dating back as far as the concept of taxonomy, when French historian Benoit de Maillet proposed that life on Earth was the result of germs "seeded" from space [source: Panspermia-Theory]. Since then, researchers from Stephen Hawking to Sir Francis Crick (who abandoned his early support for the RNA world hypothesis) have held the belief that life on Earth originated away from this planet.
The theory of panspermia falls into three broad categories. Life traveled via space debris from somewhere outside our solar system, the concept of lithopanspermia, or from another planet in our solar system, ballistic panspermia. The third hypothesis, directed panspermia, holds that life on our planet was spread purposefully by already established and intelligent life [source: Panspermia-Theory].
As panspermia hypotheses go, ballistic panspermia (also called interplanetary panspermia) enjoys the widest acceptance in the scientific community. Chunks of other planets have long bombarded Earth in the form of meteorites. In fact, one meteorite, ALH84001, discovered in Antarctica in 1984, bears what some scientists take as the traces of life or the precursors to life like amino acids. It's been calculated to have broken from Mars more than 4 billion years ago [source: Thompson].
Upon examination of ALH84001, astrobiologists -- scientists who study the potential for life in space -- found that at least four traces of ancient life, from what appeared to be fossilized microbes to a form of magnetic bacteria [source: Schirber]. Since the findings were published in 1996, three of the traces of life found in the meteorite have been discounted. But whether the last trace, chains of magnetite, are mineral or were biologically produced by ancient Martian bacteria remains under debate.
Mars is the likeliest candidate for ballistic panspermia. The arrangement of the orbits of Mars and Earth around the sun make it about 100 times easier for a rock to travel from Mars to Earth than vice versa [source: Chandler]. And over the course of Earth's history, about 5 trillion rocks are estimated to have made the journey [source: NASA]. What's more, in their early histories, Earth and Mars were similarly suited to hosting life, both featuring wet atmospheres and water on their surfaces.
Despite all of this evidence, the jury is still out on how life began on Earth. Read some criticisms of panspermia and abiogenesis on the next page.
Criticism of Abiogenesis and Panspermia
While the experiments carried out by Stanley Miller and others who have built upon his work show that life may have arisen from a primordial soup, that possibility remains theoretical. There is no evidence for pre-cellular life on Earth; what's more, critics of the RNA world hypothesis point out that the experiments that support the concepts were conducted with biologically created RNA. RNA can act as both a template for self-replication and an enzyme for carrying out that process, but these findings have been carried out in controlled laboratory experiments. This doesn't necessarily prove such delicate actions could happen in the seas of the ancient Earth.
For reasons like these, the RNA world hypothesis has been largely abandoned by proponents of abiogenesis in favor of other hypotheses, like the simultaneous development of both proteins and genetic templates or the development of life around undersea vents similar to those currently inhabited by today's extremophiles. But there is one criticism that any abiogenesis hypothesis has difficulty overcoming: time. DNA-based life is thought to have developed on Earth beginning around 3.8 billion years ago, giving pre-cellular life forms about 1 billion years to carry out random processes of encoding useful proteins and assembling them into the precursors of cellular life [source: Discovery News]. Critics of abiogenesis say that simply isn't enough time for inorganic matter to become the theorized precellular life. One estimate suggests it would take 10^450 (10 to the 450th power) years for one useful protein to be randomly created [source: Klyce].
This is one obstacle that makes panspermia an attractive explanation: It doesn't explain the origin of life, merely the origin of life on Earth. Panspermia hypotheses don't necessarily contradict abiogenesis; they merely shift the origin elsewhere. Yet the jury is still out on several important factors that must be in place for panspermia to be correct. Is it possible, for example, for microbial life to survive during the harsh conditions found in the trip through space, the entrance to Earth's atmosphere and the impact on Earth's surface?
Some recent hypotheses suggest that it needn't survive. One researcher postulates that dead scraps of DNA could have arrived on Earth via ballistic panspermia and were replicated through a kickstarted process similar to RNA world [source: Grossman]. Other researchers aim to scour Mars for fossil life and compare any genetic material to that found universally on Earth to determine relation [source: Chandler].
Yet if life on Earth did begin elsewhere and travel to our planet the question still remains: What is life's origin?
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