contents Principles of Biology 74 Early Evolution of Life Major events in early life include the evolution of prokaryotes, photosynthesis, eukaryotes, multicellularity, and the colonization of land. Alethopteris fossil. Fossilized leaves of Alethopteris sp., and extinct plant that lived in the Carboniferous period. Sinclair Stammers/Science Source. Topics Covered in this Module Early Life on Earth Major Objectives of this Module Give the date the first prokaryotes appear in the fossil record and how they were identified. Describe the geologic and biologic effects of the evolution of photosynthesis. Relate endosymbiont theory to the evolution of eukaryotes. Explain how species evolved adaptations to life on land. page 380 of 989 3 pages left in this module contents Principles of Biology 74 Early Evolution of Life When did life begin? What did the earliest life forms look like? When did plants and animals appear on Earth? Evidence for early life on Earth comes from geology and the fossil record. Early Life on Earth Scientists use radiometric dating to determine how old fossils are based on how much the radioactive isotopes they contain have decayed. The history of Earth is customarily divided into three eons: the Archaean, Proteozoic, and Phanerozoic (Figure 1). The first single-celled organisms appeared in the Archaean eon. The first eukaryotes and multicellular organisms appeared in the Proterozoic eon. Animals appeared toward the end of this eon, but most of their evolution occurred during the Phanerozoic eon, which covers approximately the last half billion years and is further divided into the Paleozoic, Mesozoic, and Cenozoic eras. Note how small a fraction of Earth's history includes humans. If the history of Earth were an hour, humans would have appeared in the last two-tenths of the last second. Figure 1: History of life on Earth.Fossils tell the history of life on Earth. The changes scientists observe in the life forms preserved as fossils are the basis of the divisions of the geologic time scale. © 2014 Nature Education All rights reserved. Figure Detail The first cells to evolve were prokaryotes. Scientists think that Earth formed with the rest of the solar system about 4.6 billion years ago. At first, the new planet would have been bombarded by rocks and ice hurtling through space, and the repeated collisions would have generated a large amount of heat. Sometime after this bombardment slowed down, the planet cooled enough that the water vapor in its atmosphere could condense, forming oceans. The atmosphere was probably made up primarily of carbon dioxide and nitrogen, but volcanic eruptions might have contributed methane, ammonia, and hydrogen sulfide. One hypothesis of the origin of life is that the first living organisms were anaerobic archaebacteria that evolved in the hydrothermal vents near submarine volcanoes, where anaerobic bacteria still live. The first evidence of life in the fossil record comes from prokaryotes: single- celled microorganisms containing DNA but no nuclei or membrane-bound organelles. Fossilized prokaryote communities bound very thin layers of sediment together into rocks called stromatolites. The oldest known stromatolites are about 3.5 billion years old. Because these fossils represent large groups of prokaryotes, the first single prokaryotes might have evolved much earlier. For about the next 1.5 billion years, they were the only life forms on the planet. Future perspectives. Prokaryotes continue to interest scientists because, despite being small, they are highly adaptable — as evidenced by the ongoing evolution of multi-drug resistant bacteria. Recent research shows that the structure of prokaryotes is more complicated than once thought. Although they do not have membrane-bound organelles, prokaryotes do have distinct subunits that perform individual functions. For instance, prokaryotes have organized structures called micro-compartments. Micro-compartments produce energy for cellular metabolism and carry out other important processes within the cell. They are made out of proteins and structurally resemble viruses. Micro-compartments might have evolved independently to look like viruses, or they may have evolved from viruses that the prokaryotes took in and lived with in symbiosis. These compartments are so small that it is difficult to examine their structure even using electron microscopy, but genetic analyses might help determine their origins. Viruses called bacteriophages are known to "infect" bacteria, but some bacteria cannot survive without phages. Phage genes encode many proteins that help bacteria defend themselves, for instance, by producing toxins. Phages increase the toxicity or the survival of several strains of bacteria, including some Escherichia coli, Streptococcus mitis, and Salmonella enterica, suggesting the two microorganisms may commonly live in mutually beneficial relationships. Photosynthesis changed the atmosphere. Oxygen makes up 21% of the atmosphere now, but when life first emerged, the atmosphere contained virtually no oxygen (Figure 2). Oxygen levels first began to rise dramatically between 2.5 to 2 billion years ago. During this time, photosynthetic prokaryotes emerged and diversified in the oceans, and they released more and more oxygen as a byproduct of photosynthesis. At first, the oxygen that these cyanobacteria produced remained dissolved in ocean water. However, eventually, the water became saturated with oxygen, so that additional oxygen produced by the photosynthesizing prokaryotes was released into the atmosphere. Other abrupt shifts in the levels of oxygen in the atmosphere occurred following the emergence of photosynthetic eukaryotes. The development of photosynthetic eukaryotes led to algae, which are the dominant photosynthesizers in the world's oceans, and eventually to land plants. Figure 2: Atmospheric oxygen levels on Earth over the last 4 billion years. This schematic represents our understanding of how oxygen in Earth's atmosphere has changed throughout the history of life and notes when organisms that photosynthesize evolved. (Modified from Xiong, J. & Bauer, C. E.. Complex evolution of photosynthesis. Annual Review of Plant Biology 53, 503-521 (2002). © 2002 Annual Reviews Modified from Xiong, J. & Bauer, C. E.. Complex evolution of photosynthesis. Annual Review of Plant Biology 53, 503-521 (2002). doi: 10.1146/annurev.arplant.53.100301.135212. Used with permission. After the dramatic increase in atmospheric oxygen, the environment became much less hospitable to the organisms that had evolved in the presence of very low levels of atmospheric oxygen. Oxygen is reactive and can damage cells and alter biochemical processes. As oxygen concentrations increased in Earth's atmosphere, a very large proportion of the prokaryotic species that thrived in anaerobic conditions went extinct, and species that were more tolerant of the new oxygenated conditions thrived. Eventually, lineages of these more tolerant organisms evolved the ability to utilize oxygen during respiration as a means to release stored chemical energy. A majority of the organisms that we are most familiar with (including ourselves) descended from these oxygen-reliant lineages. As photosynthetic prokaryotes, probably similar to cyanobacteria found on Earth today, colonized the oceans, they released more and more oxygen. Bacteria reproduce exponentially — the number of individuals doubles after every reproductive cycle — and so with their increased numbers, the oxygen on Earth rose very rapidly. At first, the oxygen that these cyanobacteria produced dissolved in the oceans. Then it became concentrated enough to react with iron, creating the red rocks we see today. Eventually, the oceans became saturated with oxygen, so that additional oxygen "gassed out" and built up in the air. Atmospheric oxygen levels rose — at first gradually, then steeply. The fastest change was probably driven by photosynthesis in eukaryotes as well as prokaryotes. Photosynthesis dramatically changed Earth's atmosphere. This dramatic "oxygen revolution" created an environment that was likely inhospitable to most of the life forms that existed on Earth at that time. So, how did the rise in atmospheric oxygen influence evolutionary history? Oxygen is reactive. In certain forms, it damages cells and enzymes. As oxygen concentrations increased in Earth's atmosphere, many prokaryotes probably died. Any cells that could use oxygen productively were more likely to survive and reproduce. This selective pressure ultimately led to the evolution of oxygen-reliant eukaryotes. Test Yourself Carbon dioxide (CO2) levels in Earth's atmosphere have increased by about 40% since 1850. On a geologic time scale, that's a dramatic change over a very short time. What were the effects of the oxygen revolution about 2.5 billion years ago? How might those relate to today’s problems with increasing CO2 in Earth's atmosphere? Submit How did eukaryotes evolve? The earliest identifiable fossil eukaryotes date from about 2.1 billion years ago. Unlike prokaryotes, eukaryotes have a membrane-bound nucleus and membrane-bound organelles. A wide variety of single-celled eukaryotes exist today including algae, yeasts, and protists, such as amoebae. A eukaryote has DNA enclosed in a nucleus, an endoplasmic reticulum that participates in protein synthesis, a cytoskeleton that allows it to change shape to engulf other cells and transport materials, and mitochondria that use oxygen and glucose to produce energy-containing molecules for the