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Home , History of Earth, History of life, Phanerozoic

contents Principles of 74 Early of Major events in early life include the evolution of , , , multicellularity, and the colonization of .

Alethopteris . Fossilized leaves of Alethopteris sp., and extinct that lived in the period. Sinclair Stammers/ Source.

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Early Life on

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 theory to the evolution of eukaryotes. Explain how evolved to life on land.

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contents Principles of Biology

74 Early Evolution of Life

When did life begin? What did the earliest life forms look like? When did and appear on Earth? Evidence for early life on Earth comes from and the fossil record.

Early Life on Earth Scientists use to determine how old are based on how much the radioactive isotopes they contain have decayed. The of Earth is customarily divided into three eons: the Archaean, Proteozoic, and (Figure 1). The first single-celled appeared in the Archaean eon. The first eukaryotes and multicellular organisms appeared in the 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 and is further divided into the , , and . Note how small a fraction of Earth's history includes . If the were an hour, humans would have appeared in the last two-tenths of the last second.

Figure 1: 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 scale. © 2014 Education All rights reserved. Figure Detail

The first cells to evolve were prokaryotes. Scientists think that Earth formed with the rest of the solar about 4.6 billion years ago. At first, the new would have been bombarded by rocks and ice hurtling through , and the repeated collisions would have generated a large amount of heat. Sometime after this bombardment slowed down, the planet cooled enough that the vapor in its could condense, forming . The atmosphere was probably made up primarily of dioxide and nitrogen, but volcanic eruptions might have contributed methane, , and 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 still live. The first evidence of life in the fossil record comes from prokaryotes: single- celled containing DNA but no nuclei or membrane-bound . Fossilized communities bound very thin layers of sediment together into rocks called . 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 for cellular and carry out other important processes within the . They are made out of and structurally resemble . 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 . 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 . 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. 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 produced remained dissolved in 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 , which are the dominant photosynthesizers in the '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 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 , 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 , 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?

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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 , such as amoebae. A has DNA enclosed in a nucleus, an endoplasmic reticulum that participates in 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 cell. The process of using oxygen and glucose to generate energy-packed molecules is called aerobic respiration. This ability would have allowed eukaryotes to take advantage of the sudden availability of oxygen after the oxygen revolution. How did eukaryotes evolve from prokaryotes? The nuclear envelope and endoplasmic reticulum resemble the in composition, and they probably evolved from the cell membrane folding in on itself. The mitochondria indicate a more complicated story.

A controversial idea becomes well accepted. It might be surprising to learn that every cell in the body contains an derived from bacteria. This idea was certainly very controversial when it was first proposed by Dr. . In 1967, Dr. Margulis wrote a paper about the origin of eukaryotes that described the endosymbiont theory of eukaryotic evolution — the theory that mitochondria (the organelles that conduct aerobic respiration in eukaryotes) evolved from free-living bacteria that had been engulfed by early cells. Scientists widely disregarded her idea at the time, and Margulis's paper was rejected by 15 journals before the Journal of Theoretical Biology published it. Her ideas stood the test of time, however, as the results of more and more experiments were conducted that supported her now widely-accepted theory. Margulis was not the first person to suggest the idea that eukaryotic organelles might have arisen through a process of endosymbiosis, but she had more evidence than earlier scientists. In the 19th century, the botanist Andreas Schimper observed division of the of green plants (the organelles that conduct photosynthesis). He noticed how they resembled cyanobacteria, the primitive free-living microorganisms that photosynthesize. In the early 1900s, the botanist Konstantin Mereschkowski proposed that chloroplasts evolved from symbiotic cyanobacteria, bacteria living inside cells to the benefit of both organisms. Around the same time, the scientist Ivan Emanuel Wallin suggested that mitochondria evolved from bacteria. Neither of these ideas received much attention at the time. But then in the 1960s, scientists used new microscopic techniques to observe the interior of mitochondria and realized that mitochondria have DNA arranged in circular . Bacterial DNA is arranged the same way. Margulis noted that mitochondria self-replicate by fission, as bacteria do, using their own DNA, which is arranged in a circular about the size of a bacterial chromosome. What other evidence did Margulis gather? Mitochondria are about the same size as bacteria. They have , enzymes, and transport systems much like those of bacteria, and they use these to make their own proteins. The mitochondria have a double membrane, and the innermost mitochondrial membrane has a composition similar to a bacterial membrane.

BIOSKILL Genomic Sequencing Informs Phylogenetic Analysis The technique of DNA sequencing had not yet been developed when Margulis published her landmark paper, but today sequencing is often used to determine how organisms are related to each other. Scientists compare the sequence of a protein in different species to look for the number of differences between them. Figure 3 shows a portion of the superoxide dismutase protein. This protein helps protect cells from the damaging effects of oxygen.

Figure 3: Comparing proteins. Mitochondria evolved from proteobacteria and chloroplasts evolved from cyanobacteria. An ancestral cell might have sequentially picked up first one and then the other.

© 2014 Nature Education All rights reserved. Transcript

Test Yourself

Explain how the data in Figure 3 support the endosymbiotic theory.

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BIOSKILL Why would early prokaryotes have engulfed other prokaryotes and continued to live symbiotically with them? These organelles produce energy-containing molecules that help the host cells function. Chloroplasts or, more generally, convert energy from to usable forms. Mitochondria convert energy from glucose and oxygen to usable forms. Either might have harnessed a previously unavailable energy source for their host cells, such as methane, which would have helped the cells function anaerobically. Engulfing a prokaryote that could make use of rising oxygen levels would have conferred a selection advantage because the host cell could survive the oxygen revolution. The theory of serial endosymbiosis proposes that multiple endosymbiotic events occurred in the same cell lineage (Figure 4).

Figure 4: Serial endosymbiosis. Serial endosymbiosis. Mitochondria evolved from proteobacteria and chloroplasts evolved from cyanobacteria.

© 2014 Nature Education All rights reserved. Transcript

Test Yourself

What kind of evidence probably indicates that photosynthetic eukaryotes evolved after other (heterotrophic) eukaryotes?

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Future perspectives and open questions. Endosymbiosis is not something that only happened once, by chance, early in evolutionary history. Eukaryotes are known to have engulfed other eukaryotes. For instance, different types of algae have evolved through this process of secondary endosymbiosis. Sequencing of species that have evolved through secondary endosymbiosis could help scientists understand how the process works. In 1981, Norman Weeden proposed that, because the (e.g., ) is not big enough to encode all the proteins the plastid uses, the plastid must have transferred many of its genes to the host cell's genome during endosymbiosis. According to one current estimate, 90% of the proteins a plastid uses are made by the host genome and imported from the cytoplasm. So the DNA an organelle carries into a host cell does not necessarily stay in one place but can infiltrate the nucleus. A 2002 study by William Martin and colleagues compared 24,990 proteins produced by the Arabidopsis, a model for scientific research, with the sequenced genomes of cyanobacteria and other microorganisms. The researchers found that genes derived from cyanobacteria make up a surprisingly large fraction of the eukaryotic genome — about 18%. The proteins these genes produce are active in cell growth, transcription, division, transport systems, and cellular metabolism. Cyanobacteria apparently contributed much more to plant genomes than only chloroplasts. If chloroplasts have inserted their DNA into host genomes, what about mitochondria? Mitochondrial DNA has been identified within the nuclear DNA of grasshoppers, shrimp, rats, cats, monkeys, and humans. How does DNA from organelles move into the host genome, and why do some genes stay in the organelle genome? How did bacterial genes shape our evolution? These questions drive areas of active research.

How did multicellularity evolve? Single-celled eukaryotes evolved into multicellular eukaryotes. The earliest known examples are multicellular algae, which appear in the fossil record about 1.2 billion years ago. Another estimate comes from DNA sequencing. Scientists estimate that, based on the amount of change among their DNA sequences, multicellular eukaryotes probably diverged from a common ancestor about 1.5 billion years ago. After the first few algae, a long gap appears in the fossil record. More complex organisms do not show up for another 500 million years. Between 575 and 535 million years ago, fossils of soft-bodied multicellular creatures emerge. They are called the biota because the fossils were found in Edicara Hill, . Some had simple circular forms and others had segments, like worms. The largest were over 3 feet (1 m) long. They probably fed on algae, strained plankton, or scavenged dead organisms. Other fossils found in the in may represent embryos or adults from extinct groups. Given the gap, though, why did it take so long for multicellular organisms to appear? Geologic evidence indicates that between 750 and 580 million years ago, Earth went through a of ice ages. At , the oceans and land were covered by sheets of ice. Most organisms would have been able to live only near deep- hydrothermal vents or perhaps in areas near the that were not covered in ice. The first multicellular organisms seem to have evolved around the time the "" melted (Figure 5). Figure 5: Multicellular fossil. This Proterozoic fossil is of one of the earliest animals. Philip Donoghue and colleagues used X-ray microscopy to show how early multicellular embryos develop. © 2006 Nature Publishing Group Donoghue, P. C. J., et al. Synchrotron X-ray tomographic microscopy of fossil embryos. Nature 442, 680-683 (2006) doi:10.1038/nature04890. Used with permission. Between 535 and 525 million years ago, during the period of the Paleozoic , an amazing diversity of life forms appears in the fossil record. Animals with hard body parts, bones, and shells show up for the first time. Many of these fossils were found in the Burgess in British Columbia, . About half of the current animal phyla arrive in the record during this time, including: : with , including and spiders , such as sea stars and sand dollars , which include all , such as , , , and . This period of sudden diversification is called the (Figure 6). Although most animals from this time look nothing like any animal on Earth today, they were the ancestors of most present-day animals. Early predators appearing in the Cambrian were longer than 3 ft (1 m) with claws to capture prey. The prey displayed defenses such as body armor and sharp spines. How could have resulted in tougher skin or sharper teeth? Figure 6: The Cambrian explosion. The Cambrian period, approximately 555 to 500 million years ago, was marked by a huge diversification of animal types. The ancestors of almost all of current animal phyla first appeared during the Cambrian, including the chordates, echinoderms, mollusks, arthropods, , and . © 2012 Nature Education All rights reserved. Figure Detail

Life eventually adapted to land. Fossils show that photosynthetic prokaryotes lived on damp land over a billion years ago, but fungi, plants, and animals did not evolve until much later — about 500 million years ago. How did organisms evolve to live successfully on land? One major feature that emerged among successful terrestrial photosynthetic organisms is that they evolved structures to avoid drying out. Many current plants have a waxy coating to limit evaporation of water, as well as a vascular system to take in water from . By 420 million years ago, land plants about 4 inches (10 cm) tall had evolved with a vascular system that transported water and nutrients up from the ground. By 370 million years ago, more complex plants with true roots and leaves appeared. Roots are specialized to anchor the plant and transport water and nutrients, and leaves are specialized for photosynthesis. How could natural selection have resulted in this specialization of cells? Fungi evolved along with plants in symbiotic relationships, like chloroplasts with host cells. Fossils show that fungi lived on the roots of plants almost as soon as plants colonized land. Most plants today harbor fungi in their root systems. The fungi help the plants absorb water and minerals, and the plants provide nutrients to the fungi. Arthropods, the group of animals that includes insects, were the first animals to colonize land. They appear in the fossil record about 420 million years ago. Next were , about 365 million years ago. Tetrapods are four-limbed animals that probably evolved from a particular group of fish. All mammals descended from tetrapods, including humans. Upright posture evolved uniquely and separately for biped tetrapods, such as hominids and birds. Arthropods and animal descended from tetrapods are now the most widespread and diverse groups of animals, including creatures as different as (among arthropods) butterflies, mosquitoes, spiders, and crabs and (among tetrapods) alligators, birds, moles, elephants, , kangaroos, dogs, and humans. The phenomenon of having a drink "go down the wrong way" (i.e., choking) appears to be a problem with human anatomy. It also represents an outcome of evolutionary history and a link to the developmental journey from embryo to fully developed human being. What is the origin of this less-than-ideal anatomy? The oral cavity to both the esophagus, the tube down which food travels to reach the stomach, and to the , the tube down which air travels to reach the . The reason these are connected is because they develop from similar tissues. The development of these tissues is linked to how these structures evolved in the first place. Understanding the evolutionary relationships between organisms, and the process through which complex structures evolved, provides insight into the anatomy, physiology, and behaviors that exist today. Mammals breathe through lungs. Air passes through the nose and throat and into the lungs, which are made up of tiny, delicate sacs called alveoli (see Figure 7). Inhaled oxygen diffuses across the alveoli into tiny surrounding blood vessels, and (CO2), a waste product of aerobic respiration, diffuses from the blood into the alveoli. When the CO2 is exhaled, muscles compress the lungs to squeeze the air out of the alveoli.

Figure 7: Bronchi and lungs. Bronchi branch off the trachea and further branch multiple times into smaller and smaller air way vessels, the bronchioles (black and gray-translucent staining). These branches are like a network of roots penetrating deep into the that facilitate exchange inside lung . In this image, some blood vessels (red), branches of the pulmonary artery, are also visible. © 2004 Nature Publishing Group Monforte-Muñoz, H., Walls, R. L. Intrapulmonary airways visualized by staining and clearing of whole-lung sections: the transparent human lung. Modern Pathology 17, 22–27 (2004) doi:10.1038/modpathol.3800003. Used with permission. The first respiratory organs were not lungs, however, but probably , the tissues fish use to breathe. Gills are thin tissues folded like a fan. A fish swallows water, pushing it out over the gills. As the water passes over the gills, it transfers oxygen to the blood, just as oxygen passes to the blood from the lungs. In the open air, gills collapse and dry out. In the lungs, the surfaces also must remain moist. A group of fish, called lungfish, has lungs as well as gills. Some of these fish live in shallow ponds, and during the dry season they burrow into the mud and breathe air. Most fish have organs called swim bladders that allow them to swallow air to make their bodies lighter and better able to float. A fossil fish called , estimated to be 375 million years old, had gills, lungs, and other characteristics that were more mammalian than fish-like: a flat skull with eyes on top of its skull, a neck, shoulder bones, and ribs. It is considered a possible "transition" fossil between lineages. Human embryos in an early of development have structures that resemble gills. The lungs first appear at day 26 of gestation as buds off the digestive tract. The alveoli form in the last 2 weeks before birth. By age 8, a child has about 300 million alveoli. Taken together, what does this evidence suggest? The lungs seem to have evolved from gills, the system fish use to take in air from swallowed water. In human embryos, the lungs form as offshoots of the digestive tract. thought swim bladders in fish had evolved into lungs, but now scientists believe swim bladders evolved from lungs. Ancient vertebrates better able to breathe air and live on land must have moved out of the water onto land. Eventually the vertebrates that lived entirely on land lost the gills they no longer needed. Some fish that live partly in mud retained both lungs and gills, and other fish that live entirely in water evolved swim bladders in addition to their gills. Mammals, birds, and all evolved different types of lungs. For instance, the lungs of birds conduct air to sacs in their bones and abdomen, and reptiles cannot breathe when food is in their mouths. Test Yourself

Provide two pieces of evidence that suggest humans and fish have a common ancestor.

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IN THIS MODULE

Early Life on Earth Summary Test Your Knowledge

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contents Principles of Biology

74 Early Evolution of Life Summary OBJECTIVE Give the date the first prokaryotes appear in the fossil record and how they were identified. Single-celled prokaryotes, the first sign of life on Earth, appear in the fossil record as stromatolites about 3.5 billion years ago. The best evidence is the stromatolites, fossilized layers of these organisms trapped in sediment.

OBJECTIVE Describe the geologic and biologic effects of the evolution of photosynthesis. Photosynthetic organisms dramatically increased atmospheric oxygen levels between 2 and 2.5 billion years ago. This change created selective pressure favoring organisms that used oxygen to produce energy-containing molecules.

OBJECTIVE Relate endosymbiont theory to the evolution of eukaryotes. Mitochondria and chloroplasts are widely believed to have evolved from bacteria that entered eukaryotic cells through endosymbiosis. These organelles not only have their own circular DNA, like bacteria, but contribute DNA to the nuclear genome, have double membranes and make their own proteins. Researchers are investigating this process and its effects.

OBJECTIVE Explain how species evolved adaptations to life on land. About 500 million years ago, some organisms moved from the oceans to land. Based on fossil, developmental, and anatomical evidence, scientists think humans and other mammals evolved from fish and lungs evolved from gills.

Key Terms endosymbiotic theory Widely accepted theory that states mitochondria and chloroplasts were once free-living prokaryotes prior to incorporation inside cells. serial endosymbiosis Theory that multiple endosymbiotic events occur in the same cell.

References Archibald, J. M. & Keeling, P. J. Plant genomes: cyanobacterial genes revealed. Heredity 90, 2–3 (2003). doi:10.1038/sj.hdy.6800204 (article)

Effros, R. M. Anatomy, development, and physiology of the lungs. GI online (2006). doi:10.1038/gimo73 (article) Hunter, P. Not so simple after all: A of research into prokaryotic evolution and cell structure. EMBO reports 9, 224–226 (2008). doi:10.1038/embor.2008.24 (article)

Kump, L. R. The rise of atmospheric oxygen. Nature 451, 277–278 (2008). doi:10.1038/nature06587 (link)

Maxmen, A. Friendly bacteria fight the flu. Nature (2011). doi:10.1038/news.2011.159 (link) Steinman, M. & Hill, R. Sequence Homologies Among Bacterial and Mitochondrial Superoxide Dismutases. PNAS 70, 3725–3729 (1973). (link)

Timmis, J. N. et al. Endosymbiotic gene transfer: Organelle genomes forge eukaryotic chromosomes. 5, 123–135 (2004). doi:10.1038/nrg1271 (article)

Vargas-Parada, L. Mitochondria and the immune response. Nature Education 3, 15 (2010). (link)

IN THIS MODULE

Early Life on Earth Summary Test Your Knowledge

page 382 of 989 1 pages left in this module

contents Principles of Biology

74 Early Evolution of Life

Test Your Knowledge

1. When do the first cells appear in the fossil record?

1.2 billion years ago 2.3 billion years ago 535 million years ago 4.6 billion years ago 3.5 billion years ago

2. How did photosynthetic prokaryotes dramatically change Earth's atmosphere?

They increased carbon dioxide levels by burning fossil fuels. They increased carbon dioxide levels through respiration. They increased oxygen levels through photosynthesis. They reduced nitrogen levels by fixing nitrogen. They reduced methane levels through anaerobic respiration.

3. Which of the following is NOT a way in which mitochondria and bacteria are similar?

Mitochondrial genes resemble proteobacterial genes. Their ribosomal structures and RNA are similar. They both produce all the proteins they need. They are similar in size. They both have DNA arranged in circular chromosomes.

4. After Lynn Margulis's endosymbiont theory met opposition, what new evidence emerged in support of the theory?

composition analysis light microscopy electron microscopy DNA sequencing transitional fossils

5. What is the main distinction between prokaryotes and eukaryotes?

Eukaryotes have chloroplasts, and mitochondria and prokaryotes have neither. Only eukaryotes have ribosomes. Eukaryotes have a nuclear membrane. Eukaryotes are multicellular; prokaryotes are unicellular. Eukaryotes have more DNA and are therefore more complex than prokaryotes.

6. Which of the following would NOT likely have occurred without the evolution of photosynthetic organisms?

Life would be much less diverse. Most organisms would have gone extinct due to the increase in oxygen. Mitochondria-containing cells would not have evolved. There would be no oxygen on Earth. All of the other choices are potential consequences of a photosynthesis-free world. Submit

IN THIS MODULE

Early Life on Earth Summary Test Your Knowledge

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