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The Appearance of The First Cell
It is not too far of a theoretical jump from the protein/RNA co-evolution explored in the previous article to a circular self-replicating single-stranded RNA sequence a few hundred nucleotides long. This is, in essence, a viroid in our modern ecosystem. But it is not a cell and according to most experts it is not alive. Life makes its indisputable appearance when the first cells appear on Earth. Biochemical activity is now confined and protected from the outside elements by a membrane and/or cell wall. The first simple cells would have evolved in a world where viroids (open genetic material) and virus-like entities (genetic material enclosed in a protein coat) exchanged genetic material between themselves and between the first simple cells to evolve, such as archaea and bacteria. This process is called lateral gene transfer and it could account for the acquisition of new biochemical pathways in microbes. The opportunity to acquire various new defensive chemical arsenals might also have allowed these first simple cells to survive the rapidly changing harsh conditions prevalent on our young planet at the time.
The first simple cell membrane might have been a simple bilayer phospholipid vesicle, a hollow spherical shell structure. Chemical pathways responsible for the pre-biotic formation of phospholipids are fairly well understood. When phospholipids (which have hydrophobic or water-hating tails) are placed in water they spontaneously form vesicles where the tails face inward. The gradual evolution of a more complex cell membrane (equipped with channels and cell pumps) and, for some cells, an even more protective cell wall would follow. As archaea and bacteria make their first appearance, Benal's third stage of evolution toward life - from complex biomolecules, like proteins and RNA, to cells - would be achieved. Modern archaea micro-organisms such as the simple unicellular organisms that live around hydrothermal vents and provide part of the food chain base there, might resemble what the first forms of life on Earth looked like. Archaea have the simplest life plan on this planet. They look like bacteria but they are biochemically very different. The chemistry of their bilayer phospholipid membranes is unique. It contains ether bonds that are more chemically resistant and heat-stable than those in either bacterial or eukaryotic cells. Eukaryotic cells are the kind of cells that make us up – with the exception of the extensive microbial flora in our guts, which consist of bacteria, fungi and archaea. Our relationship with unicellular life is even more intimate and intermingled than hosting microbes in our guts. Extensive evidence suggests that genetic vestiges of ancient unicellular microbes are present in each and every eukaryotic cell in our bodies as well. Our cells contain a complex mosaic of genetic material that was obtained by genetic exchanges between ancient eukaryotic micro-organisms, bacteria and archaea. In addition to lateral gene transfer, ancient cells also likely went one step further by simply engulfing other cells and eventually utilizing their unique cellular machinery as organelles. This evolutionary process, called endosymbiosis, may be responsible for the appearance of the first (organelle-containing) eukaryotic micro-organism. Symbiogenesis is the theory that various organelles inside our eukaryotic cells originated from symbiotic (cooperative) relationships between different strains of ancient archaea and bacteria. There is strong evidence that mitochondria, the "powerhouses" of our cells where ATP is produced, are of bacterial origin. Those ancient bacteria were likely engulfed and incorporated into a eukaryotic predecessor.
Tough Intrepid Archaea
Archaea are especially interesting from a life origin point of view because they are the most likely candidates to handle the extreme conditions on our young planet. Tough ether membrane bonds explain why many archaea are extremophiles, able to live in environments far too harsh for other organisms. Most archaea also possess a unique protein cell wall, which makes them even tougher and which further differentiates them from bacteria. Archaea possess genes and metabolic pathways that closely resemble those of eukaryotes (again suggesting that eukaryotes borrowed these useful traits from archaea) but, unlike eukaryotes and like bacteria, archaea don't have any internal structure such as organelles. A single circular strand of DNA and a few independent DNA pieces called plasmids float inside an amorphous cytoplasm. DNA transfer between cells is common and viruses can infect them as well. These sources of new DNA promote rapid evolution in times of hardship, and make ancient symbiotic relationships easy to visualize. The wide variety of chemical reactions that take place inside these tiny cells is really what sets them apart and this is what ultimately made them so wildly successful, allowing them to inhabit virtually every possible location on Earth and is what allowed this life domain to exist longer than any known living organism. This chemical variety also enables archaea to utilize many different sources of energy. This makes them a prime candidate to look for on other planets and moons where a carbon-based biochemistry could also evolve.
Archaea's unique biochemistry suggests that these organisms evolved independently from bacteria, even though they share the same basic genetic structure – a single circular strand of DNA and possibly plasmids as well. The shared structure of circular DNA means that the origin of DNA probably predates the separation of archaea and bacteria into two significantly different evolutionary domains. Chemical fossils of archaea's unique lipids were found in some of Earth's oldest known sedimentary rock in Greenland, which is dated to 3.8 billion years old. This supports increasing evidence that archaea was Earth's first living organism. Archaea might also be responsible for 4.1 billion year old carbon isotope chemical fossils indicating a life process, mentioned in a previous article in this series. However, this is evidence only for carbon-based life, not for any specific life domain.
Archaea, Bacteria and Eukaryotes: A Rich Tapestry of Earth Biochemistry
While archaea stands out as being the best candidate for surviving deep underground when Earth's surface was far too hot and violent for life, aggregates of both modern archaea and modern bacteria behave in additional ways that make them both ultimate survivors. They can transfer genes laterally among one another and they can undergo recombination (gene mixing) at rates far higher than more complex eukaryotic unicellular organisms can. One can guess, with so much genetic variation available, that these organisms could adapt remarkably well and quickly to the dramatically ever-changing conditions on early Earth, especially on the surface. As conditions moderated over millions of years, variants that could utilize the Sun's ultraviolet light for energy evolved, leading eventually to the first simple photosynthetic biochemical pathways, probably in cyanobacteria. Oxygen, the waste gas of photosynthesis, oxidized iron in rock and was absorbed by organic material. Eventually, it built up in the atmosphere. Toxic to anaerobes (which includes many archaea and bacteria), atmospheric oxygenation not only kicked off one of Earth's most significant extinction events, it reacted with atmospheric methane, a potent greenhouse gas, triggering the longest global glaciation period in Earth's history. Despite the catastrophe, life persisted and aerobic organisms (those that require oxygen to live) evolved. Oxygen made it energetically possible for complex multicellular highly mobile organisms such as us to evolve. The electrochemical transport chain of cellular espiration in our cells uses oxygen to metabolize molecules such as high-energy sugars, a process which yields more energy than fermentation or anaerobic respiration. The downside of oxygen-based metabolism is the oxidative stress placed on cells. Oxygen is a very reactive molecule so peroxides and free radicals, which damage proteins and DNA, build up in cells. Cells have evolved various defense mechanisms to eliminate the destructive molecules and DNA and proteins are constantly repaired, at some metabolic cost (this is one reason why we age and die).
Obtaining Energy: Survivors Versus Specialists
Earth's biosphere boasts three different methods for carbon-based organisms to obtain energy (cellular respiration), a key and universal requirement of life: anaerobic respiration, fermentation and aerobic respiration. Each has its own advantages and disadvantages, and all three are required for complex organisms like us to survive. Many unicellular fungi (yeasts) and bacteria utilize the simple process of fermentation to obtain energy. There is no complex electrochemical gradient involved. The simplest reactions turn sugars into alcohols. The production of bread, beer, wine and cheese all require fermentation. Ruminants such as cattle, goats and deer have evolved long guts full of bacteria optimized to ferment the otherwise indigestible cellulose in grasses, bark and twigs.
Fermentation also functions as a "plan B" in the metabolism of some of our mammalian tissues. For example, our muscle cells turn to fermentation when they are not getting enough oxygen to function, as during a long strenuous workout when glucose stored in the muscle cells is used up. Fermentation produces lactic acid as a cellular waste product and that makes our muscles feel sore and stiff afterward. Our bodies are specialized for optimal performance over a narrow range of conditions, such as temperature, food, and the right mixture of gases to breathe. This energy efficiency has allowed our large curious energy-draining brains to evolve. Microbes such as archaea trade efficiency for survivability under great and unpredictable environmental stresses. For example, bacteria and archaea survived for millions of years tucked away in areas devoid of oxygen while Earth's surface remained frozen solid. These organisms, though tough and versatile, have a much less efficient electrochemical gradient than oxygen-using aerobes. In the anaerobic electrochemical transport chain, less oxidizing substances such as sulphates, nitrates and sulphur are used instead of oxygen. Less oxidation = less available metabolic energy. On a very stable planetary environment complex organisms like us could excel but in unstable conditions, microbes will likely win the life game. These differences on Earth offer clues to what kinds of life we could expect to detect on various exoplanets based on their geology and climate.
Unique Geological History = Unique Planetary Biosphere
Simple unicellular organisms, though not winners in the energy game, are winners at long-term survival. Dwelling in soil and under water, they make life for multicellular organisms such as us possible. They are key drivers of the carbon and nitrogen cycles, and they break down dead organic matter and remove heavy metals from solution in water. Life evolved under a great variety of environmental pressures, creating a great range of biochemical adaptations. This rich variety is what our complex modern biosphere is based upon. How likely is it that such a variety of unicellular life evolved under harsh and rapidly changing conditions on another world? A planet's unique biochemical variety might depend on the changing conditions in which it evolved geologically. Is a wide variety of unicellular life necessary for more complex multicellular life, intelligent technology-bearing life like us, to evolve? How many worlds have life that is restricted to a simple palette of a few different but tough unicellular plans?
Unicellular Life Might Be Plentiful In the Universe
Although knowledge about our evolutionary progression from pre-life chemistry to simple unicellular life is not yet seamless, research in many areas is beginning to fit enough pieces on the table to glimpse what life's beginnings might look like. Any of the three carbon-based biochemical energy pathways described above (and more) could evolve on other worlds if a variety of organic molecules are present along with liquid water and available energy. By looking closely at the evolution of archaea and bacteria on Earth, we get the sense that at least simple unicellular carbon-based life could evolve even in very different and very hostile environments.
The origin of proteins, RNA and DNA explored in the previous article in this series does not mean that other completely unique kinds of biochemistry couldn't develop into possibly very complex living organisms on other planets. It only means that Earth's general biochemistry is the one that won out over time here. Exotic biochemistries might exist on exoplanets, perhaps under temperature or pressure extremes not encountered on Earth, utilizing biological solvents other than water and deriving energy from a star unlike the Sun. However, based on what we know about Earth's history, archaea-like carbon-based life, and life evolved from archaea-like ancestors seems to be a good bet, at least on planets with liquid water. One can argue that an archaea-like last universal ancestor evolved into our unique complex multicellular life as a response to Earth's unique geological evolution. Who knows how a similar unicellular ancestor might evolve on an exoplanet where geological evolution veered off in another direction?
We've explored the past - how life came about on our once young and very violent planet. Next we will look at what the future holds. Scientists are working on sophisticated technologies that will get us a closer and more intimate look at the exoplanets we are discovering on almost a daily basis.