An irony of fossilization is that conditions hostile to life usually , because nothing disturbed the sediments, which were and often sulfidic. In the sea sediments that mark the geologic periods, white limestone and black shale are typical layers. Limestone means oxygenated oceans, and black shales and mudstones mean anoxic conditions. The black color means carbon, as the ecosystems could not recycle the carbon and it was instead preserved into the sediments which have been the burned in today’s industrialized world.
Eyes began with that captured photons that through chemical cycles in a new kind of specialized cell: the nerve cell. Neurons are energy hogs and “high-tension electric lines” in animals. Human brain tissue uses ten times the energy that non-organ tissues elsewhere in the body do. The first eyes probably only detected light, and perhaps even infrared light, so that from life-giving/destroying volcanic vents, for instance. Hydrothermal vent shrimp today , which can be likened to naked retinas. The development of an eye with a lens was not a great evolutionary leap from rudimentary eyes, and a recent calculation shows how eyes with lenses could have developed from scratch in about a half-million years of evolution. may have had the first precursors to eyes. Once the eye evolved, its benefit was overwhelmingly obvious, and virtually all animals that live where vision would help them have eyes. Animals that adopted subterranean existences . It is thought today that the development of eyes was a key innovation in the arms race that would soon characterize the eon of animals, and might have even triggered it. The gene is common to all animals with eyes. As , that gene supports the widely accepted idea that . The purpose of all senses is to detect environmental information, which is in turn processed by the brain. Even brainless plants can detect light and modify their behavior, such as .
In summary, today’s orthodox late-Proterozoic hypothesis is that the complex dynamics of a supercontinent breakup somehow triggered . The global glaciation was reversed by runaway effects primarily related to an immense increase in atmospheric carbon dioxide. During the events, oceanic life would have been delivered vast amounts of continental nutrients scoured from the rocks by glaciers, and the hot conditions would have combined to create a global explosion of photosynthetic life. A billion years of relative equilibrium between prokaryotes and eukaryotes was ultimately shattered, and oxygen levels began rising during the Cryogenian and Ediacaran periods toward modern levels. Largely sterilized oceans, which began to be oxygenated at depth for the first time, are now thought to have prepared the way for what came next: the rise of complex life.
Part of the hypothesis for skyrocketing oxygen levels during the late Proterozoic was that high carbon dioxide levels, combined with a continent that had been ground down by glaciers, and the resumption of the hydrological cycle, which would have vanished during the Snowball Earth events, would have created conditions of dramatically increased erosion, which would have buried carbon (the cap carbonates are part of that evidence) and thus helped oxygenate the atmosphere. Evidence for that increased erosion also came in the form of strontium isotope analysis. Two of strontium’s stable isotopes are . Earth’s mantle is enriched in strontium-86 while the crust is enriched in strontium-87, so basalts exposed to the ocean in the oceanic volcanic ridges are enriched in strontium-86 while continental rocks are enriched in strontium-87. If erosion is higher than normal, then ocean sediments will be enriched in strontium-87, which analysis of Ediacaran sediments confirmed. That evidence, combined with carbon isotope ratios, provides a strong indication of high erosion and high carbon burial, which would have increased atmospheric oxygen levels. There is other evidence of increasing atmospheric oxygen content during the late Proterozoic, such as an increase in rare earth elements in Ediacaran sediments. Although there is still plenty of controversy, today's consensus is that the Cryogenian is when , where they have largely stayed, although as this essay will later discuss, oxygen levels have varied widely since the late Proterozoic (from perhaps only a few percent to 35%).
For this essay’s purposes, the most important ecological understanding is that the Sun provides all of earthly life’s energy, either (all except nuclear-powered electric lights driving photosynthesis in greenhouses, as that energy came from dead stars). Today’s hydrocarbon energy that powers our industrial world comes from captured sunlight. Exciting electrons with photon energy, then stripping off electrons and protons and using their electric potential to power biochemical reactions, is what makes Earth’s ecosystems possible. Too little energy, and reactions will not happen (such as ice ages, enzyme poisoning, the darkness of night, food shortages, and lack of key nutrients that support biological reactions), and too much (such as , ionizing radiation, temperatures too high for enzyme activity), and life is damaged or destroyed. The journey of life on Earth has primarily been about adapting to varying energy conditions and finding levels where life can survive. For the many hypotheses about those ancient events and what really happened, the answers are always primarily in energy terms, such as how it was obtained, how it was preserved, and how it was used. For life scientists, that is always the framework, and they devote themselves to discovering how the energy game was played.
The critical feature of earliest life had to be a way to reproduce itself, and is common to all cellular life today. The DNA that exists today was almost certainly not a feature of the first life. The most accepted hypothesis is that . The mechanism today is that DNA makes RNA, and RNA makes proteins. DNA, RNA, proteins, sugars, and fats are the most important molecules in life forms, and very early on, protein “learned” the most important trick of all, which was an energy innovation: facilitate biological reactions. If we think about at the molecular level, it is the energy that crashes molecules into each other, and if they are crashed into each other fast enough and hard enough, the reaction becomes more likely. But that is an incredibly inefficient way to do it. It is like putting a key in a room with a lock in a door and shaking up the room in the hope that the key will insert itself into the lock during one of its collisions with the room’s walls. Proteins make the process far easier, and those proteins are called enzymes.
There is , but it is currently thought that life on Earth today descended from organism, a creature known today as the Last Universal Common Ancestor (“”). The reasoning is partly that all life has a preference for using certain types of molecules. Many molecules with the same atomic structure can form mirror images of themselves. That mirror-image phenomenon is called . In nature, such mirror images occur randomly, but life prefers one mirror image over the other. In all life on Earth, proteins are virtually without exception left-handed, while sugars are right-handed. If there was more than one line of descent, life with different “handedness” would be expected, but it has never been found, which has led scientists to think that LUCA is the only survivor that spawned all life on Earth today. All other lineages died out (the likely answer, and there was probably hundreds of millions of years of evolution on Earth before LUCA lived), or they may have all descended from the same original organism. As we will see, this is far from the only instance when such seminal events are considered to have probably happened only . Also, the unique structure of DNA and many enzymes are common to all life, and they did not have to form the way that they did. That they came through different ancestral lines is unlikely.
The bundle-sheath cell membrane is impermeable to CO2 which results in a higher concentration of CO2 within the cell which is where the Calvin Cycle occurs.
Enzymes speed up chemical reactions and they do it as in the above analogy but as if a person entered that room, picked up the key, and inserted it into the lock. That took far less effort than shaking up the room a million times. Enzymes are like hands that grab two molecules and bring them into alignment so that the key inserts into the lock. The is the standard way to explain enzymes to non-scientists. Enzymes make chemical reactions happen millions and even billions of times faster than they would occur in the enzymes’ absence. Life would never have grown beyond some microscopic curiosities without the assistance that enzymes provide. Almost all enzymes are proteins, which are generally huge molecules with intricate folds. The animation of human glyoxalase below depicts a standard (author is at , and the zinc ions that make it work are the purple balls).
In the earliest days of life on Earth, it had to solve the problems of how to reproduce, how to separate itself from its environment, how to acquire raw materials, and how to make the chemical reactions that it needed. But it was confined to those areas where it could take advantage of briefly available potential energy as . The earliest process of skimming energy from energy gradients to power life is called respiration. That earliest respiration is today called because there was virtually no free oxygen in the atmosphere or ocean in those early days. Respiration was life’s first energy cycle. A biological energy cycle begins by harvesting an energy gradient (usually by a proton crossing a membrane or, in photosynthesis, directly capturing photon energy), and the acquired energy powered chemical reactions. The cycle then proceeds in steps, and the reaction products of each step sequentially use a little more energy from the initial capture until the initial energy has been depleted and the cycle’s molecules are returned to their starting point and ready for a fresh influx of energy to repeat the cycle.
In photorespiration, oxygen reacts with the ribulose-1,5-bisphosphate to reverse carbon fixation in the Calvin Cycle and thus, reduces the efficiency of photosynthesis.