Chapter 26   The Tree of Life:
An Introduction to Biological Diversity

Lecture Outline

Overview: Changing Life on a Changing Earth

·        Life is a continuum extending from the earliest organisms to the great variety of forms alive today.

·        Organisms interact with their environments.

°        Geological events that alter environments change the course of biological history.

§        When glaciers recede and the land rebounds, marine creatures can be trapped in what gradually become freshwater lakes.

§        Populations of organisms trapped in these lakes are isolated from parent populations, and may evolve into new species.

°        Life changes the planet it inhabits.

§        The evolution of photosynthetic organisms released oxygen into the air, with a dramatic effect on Earth’s atmosphere.

°        The emergence of Homo sapiens has changed the land, water, and air at an unprecedented rate.

·        Historical study of any sort is an inexact discipline that depends on the preservation, reliability, and interpretation of records.

°        The fossil record of past life is generally less and less complete the further into the past we delve.

°        Fortunately, each organism alive today carries traces of its evolutionary history in its molecules, metabolism, and anatomy.

°        Still, the evolutionary episodes of greatest antiquity are generally the most obscure.

 

Concept 26.1 Conditions on early Earth made the origin of life possible

·        Most biologists now think that it is credible that chemical and physical processes on Earth produced simple cells.

·        According to one hypothetical scenario, there were four main stages in this process:

1.     The abiotic synthesis of small organic molecules (monomers).

2.     The joining of monomers into polymers.

3.     The packaging of these molecules into protobionts, droplets with membranes that maintained a distinct internal chemistry.

4.     The origin of self-replicating molecules that eventually made inheritance possible.

·        The scenario is speculative but does lead to predictions that can be tested in laboratory experiments.

·        Earth and the other planets in the solar system formed about 4.6 billion years ago, condensing from a vast cloud of dust and rocks surrounding the young sun.

·        It is unlikely that life could have originated or survived in the first few hundred million years after the Earth’s formation.

°        The planet was bombarded by huge bodies of rock and ice left over from the formation of the solar system.

°        These collisions generated enough heat to vaporize all available water and prevent the formation of the seas.

·        The oldest rocks on the Earth’s surface, located at a site called Isua in Greenland, are 3.8 billion years old.

°        It is not clear whether these rocks show traces of life.

 The first cells may have originated by chemical evolution on a young Earth.

·        It is credible that chemical and physical processes on early Earth produced the first cells.

·        According to one hypothesis, there were four main stages to this process:

1.     Abiotic processes synthesized small organic molecules, such as amino acids and nucleotides.

2.     These monomers were joined into polymers, including proteins and nucleic acids.

3.     Polymers were packaged into “protobionts,” droplets with membranes that maintained an internal chemistry distinct from their surroundings.

4.     Self-replicating molecules arose, making inheritance possible.

 Abiotic synthesis of organic monomers is a testable hypothesis.

·        As the bombardment of early Earth slowed, conditions on the planet were very different from today.

°        The first atmosphere may have been a reducing atmosphere thick with water vapor, along with nitrogen and its oxides, carbon dioxide, methane, ammonia, hydrogen, and hydrogen sulfide.

°        Similar compounds are released from volcanic eruptions today.

·        As Earth cooled, the water vapor condensed into the oceans and much of the hydrogen was lost into space.

·        In the 1920s, Russian chemist A. I. Oparin and British scientist J. B. S. Haldane independently postulated that conditions on early Earth favored the synthesis of organic compounds from inorganic precursors.

°        They reasoned that this could not happen today because high levels of oxygen in the atmosphere attack chemical bonds.

°        A reducing environment in the early atmosphere would have promoted the joining of simple molecules to form more complex ones.

·        The considerable energy required to make organic molecules could be provided by lightning and the intense UV radiation that penetrated the primitive atmosphere.

°        Young suns emit more UV radiation. The lack of an ozone layer in the early atmosphere would have allowed this radiation to reach Earth.

·        Haldane suggested that the early oceans were a solution of organic molecules, a “primitive soup” from which life arose.

·        In 1953, Stanley Miller and Harold Urey tested the Oparin-Haldane hypothesis by creating, in the laboratory, the conditions that had been postulated for early Earth.

·        They discharged sparks in an “atmosphere” of gases and water vapor.

·        The Miller-Urey experiments produced a variety of amino acids and other organic molecules.

°        Other attempts to reproduce the Miller-Urey experiment with other gas mixtures have also produced organic molecules, although in smaller quantities.

·        It is unclear whether the atmosphere contained enough methane and ammonia to be reducing.

°        There is growing evidence that the early atmosphere was made up primarily of nitrogen and carbon dioxide.

°        Miller-Urey-type experiments with such atmospheres have not produced organic molecules.

§        It is likely that small “pockets” of the early atmosphere near volcanic openings were reducing.

·        Alternate sites proposed for the synthesis of organic molecules include submerged volcanoes and deep-sea vents where hot water and minerals gush into the deep ocean.

°        These regions are rich in inorganic sulfur and iron compounds, which are important in ATP synthesis by present-day organisms.

·        Some of the organic compounds from which the first life on Earth arose may have come from space.

·        Researchers are looking outside of Earth for clues about the origin of life.

°        Evidence is growing that Mars was relatively warm for a brief period, with liquid water and an atmosphere rich in carbon dioxide.

°        During that period, prebiotic chemistry similar to that on early Earth may have occurred on Mars.

°        Did life evolve on Mars and then die out, or did dropping temperatures and a thinning atmosphere terminate prebiotic chemistry before life evolved?

°        Liquid water lies beneath the ice-covered surface of Europa, one of Jupiter’s moons, raising the possibility that Europa’s hidden ocean may harbor life.

°        Detection of free oxygen in the atmosphere of any planets outside our solar system would be strongly suggestive of oxygenic photosynthesis.

 Laboratory simulations of early-Earth conditions have produced organic polymers.

·        The abiotic origin hypothesis predicts that monomers should link to form polymers without enzymes and other cellular equipment.

·        Researchers have produced polymers, including polypeptides, after dripping solutions of monomers onto hot sand, clay, or rock.

°        Similar conditions likely existed on early Earth at deep-sea vents or when dilute solutions of monomers splashed onto fresh lava.

 Protobionts can form by self-assembly.

·        Life is defined by two properties: accurate replication and metabolism.

°        Neither property can exist without the other.

·        DNA molecules carry genetic information, including the information needed for accurate replication.

°        The replication of DNA requires elaborate enzymatic machinery, along with a copious supply of nucleotide building blocks provided by cell metabolism.

·        Although Miller-Urey experiments have yielded some of the nitrogenous bases of DNA and RNA, they have not produced anything like nucleotides.

°        Thus, nucleotides were likely not part of the early organic soup.

·        Self-replicating molecules and a metabolism-like source of the building blocks must have appeared together.

°        The necessary conditions may have been provided by protobionts, aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure.

°        Protobionts exhibit some of the properties associated with life, including reproduction and metabolism, and can maintain an internal chemical environment different from their surroundings.

·        Laboratory experiments show the spontaneous formation of protobionts from abiotically produced organic compounds.

°        For example, droplets of abiotically produced organic compounds called liposomes form when lipids and other organic molecules are added to water.

°        The lipids form a molecular bilayer at the droplet surface, much like the lipid bilayer of a membrane.

°        These droplets can undergo osmotic swelling or shrinking in different salt concentrations.

°        Some liposomes store energy in the form of a membrane potential.

·        Liposomes behave dynamically, growing by engulfing smaller liposomes or “giving birth” to smaller liposomes.

·        If similar droplets forming in ponds on early Earth incorporate random polymers of linked amino acids into their membranes, and if some of these polymers made the membranes permeable to molecules, then those droplets could have selectively taken up organic molecules from their environment.

 RNA may have been the first genetic material.

·        The first genetic material was probably RNA, not DNA.

°        Thomas Cech and Sidney Altman found that RNA molecules not only play a central role in protein synthesis, but also are important catalysts in modern cells.

·        RNA catalysts, called ribozymes, remove their own introns and modify tRNA molecules to make them fully functional.

°        Ribozymes also help catalyze the synthesis of new RNA polymers.

°        Ribozyme-catalyzed reactions are slow, but the proteins normally associated with ribozymes can increase the reaction rate more than a thousandfold.

·        Laboratory experiments have demonstrated that RNA sequences can evolve under abiotic conditions.

°        Unlike double-stranded DNA, single-stranded RNA molecules can assume a variety of 3-D shapes specified by their nucleotide sequences.

°        RNA molecules have both a genotype (nucleotide sequence) and a phenotype (three-dimensional shape) that interacts with surrounding molecules.

°        Under particular conditions, some RNA sequences are more stable and replicate faster and with fewer errors than other sequences.

°        Occasional copying errors create mutations; selection screens these mutations for the most stable or the best at self-replication.

°        Beginning with a diversity of RNA molecules that must compete for monomers to replicate, the sequence best suited to the temperature, salt concentration, and other features of the surrounding environment and having the greatest autocatalytic activity will increase in frequency.

°        Its descendents will be a family of closely related RNA sequences, differing due to copying errors.

°        Some copying errors will result in molecules that are more stable or more capable of self-replication.

°        Similar selection events may have occurred on early Earth.

·        Modern molecular biology may have been preceded by an “RNA world.”

 Natural selection could refine protobionts containing hereditary information.

·        The first RNA molecules may have been short, virus-like sequences, aided in their replication by amino acid polymers with rudimentary catalytic capabilities.

°        This early replication may have taken place inside protobionts.

°        RNA-directed protein synthesis may have begun as weak binding of specific amino acids to bases along RNA molecules, which functioned as simple templates holding a few amino acids together long enough for them to be linked.

°        This is one function of rRNA today in ribosomes.

·        Some RNA molecules may have synthesized short polypeptides that behaved as enzymes helping RNA replication.

°        Early chemical dynamics would include molecular cooperation as well as competition.

·        Other RNA sequences might have become embedded in the protobiont membrane, allowing it to use high-energy inorganic molecules such as hydrogen sulfide to carry out organic reactions.

·        A protobiont with self-replicating, catalytic RNA would differ from others without RNA or with RNA with fewer capabilities.

·        If that protobiont could grow, split, and pass its RNA molecules to its daughters, the daughters would have some of the properties of their parent.

°        The first protobionts must have had limited amounts of genetic information, specifying only a few properties.

°        Because their properties were heritable, they could be acted on by natural selection.

·        The most successful of these protobionts would have increased in numbers, because they could exploit available resources and produce a number of similar daughter protobionts.

·        Once RNA sequences that carried genetic information appeared in protobionts, many further changes were possible.

°        One refinement was the replacement of RNA as the repository of genetic information by DNA.

°        Double-stranded DNA is a more stable molecule, and it can be replicated more accurately.

°        Once DNA appeared, RNA molecules would have begun to take on their modern roles as intermediates in translation of genetic programs.

°        The “RNA world” gave way to a “DNA world.”

 

Concept 26.2 The fossil record chronicles life on Earth

 Radiometric dating gives absolute dates for some rock strata.

·        The relative sequence of fossils in rock strata tells us the order in which the fossils were formed, but it does not tell us their ages.

·        Geologists have developed methods for obtaining absolute dates for fossils.

·        One of the most common techniques is radiometric dating, which is based on the decay of radioactive isotopes.

°        An isotope’s half-life, the number of years it takes for 50% of the original sample to decay, is unaffected by temperature, pressure, or other environmental variables.

·        Fossils contain isotopes of elements that accumulated while the organisms were alive.

°        For example, the carbon in a living organism contains the most common carbon isotope, carbon-12, as well as a radioactive isotope, carbon-14.

°        When an organism dies, it stops accumulating carbon, and the carbon-14 that it contained at the time of death slowly decays to nitrogen-14.

°        By measuring the ratio of carbon-14 to total carbon or to nitrogen-14 in a fossil, we can determine the fossil’s age.

§        With a half-life of 5,730 years, carbon-13 is useful for dating fossils up to about 75,000 years old.

§        Fossils older than that contain too little carbon-14 to be detected by current techniques.

°        Radioactive isotopes with longer half-lives are used to date older fossils.

°        Paleontologists can determine the age of fossils sandwiched between layers of volcanic rocks by measuring the amount of potassium-40 in those layers.

°        Potassium-40 decays to the chemically unreactive gas argon-40, which is trapped in the rock.

§        When the rock is heated during a volcanic eruption, the argon is driven out, but the potassium remains.

§        This resets the clock for potassium-40 to zero.

°        The current ratio of potassium-40 to argon-40 in a layer of volcanic rock gives an estimate of when that layer was formed.

°        Magnetism of rocks can also be used to date them.

°        When volcanic or sedimentary rock forms, iron particles in the rock align themselves with Earth’s magnetic field.

§        When the rock hardens, their orientation is frozen in time.

§        Geologists have determined that Earth’s north and south magnetic poles have reversed repeatedly in the past.

§        These magnetic reversals have left their record on rocks throughout the world.

à       Patterns of magnetic reversal can be matched with corresponding patterns elsewhere, allowing rocks to be dated when other methods are not available.

 Geologists have established a geologic record of Earth’s history.

·        By studying rocks and fossils at many different sites, geologists have established a geologic record of the history of life on Earth, which is divided into three eons.

·        The first two eons—the Archaean and the Proterozoic—lasted approximately four billion years.

°        These two eons are referred to as the Precambrian.

·        The Phanerozoic eon covers the last half billion years and encompasses much of the time that multicellular eukaryotic life has existed on Earth.

°        It is divided into three eras: Paleozoic, Mesozoic, and Cenozoic.

°        Each age represents a distinct age in the history of Earth and life on Earth.

°        The boundaries between eras correspond to times of mass extinction, when many forms of life disappeared.

 Mass extinctions have destroyed the majority of species on Earth.

·        A species may become extinct for many reasons.

°        Its habitat may have been destroyed, or its environment may have changed in a direction unfavorable to the species.

°        Biological factors may change, as evolutionary changes in one species impact others.

·        On a number of occasions, global environmental changes were so rapid and major that the majority of species went extinct.

°        Such mass extinctions are known primarily from the loss of shallow-water, marine, hard-bodied animals, the organisms for which the fossil record is most complete.

·        The Permian mass extinction defines the boundary between the Paleozoic and Mesozoic eras.

°        Ninety-six percent of marine animal species went extinct in less than 5 million years.

°        Terrestrial life was also affected.

·        The Cretaceous extinction of 65 million years marks the boundary between the Mesozoic and Cenozoic eras.

°        More than half of all marine species and many families of terrestrial plants and animals, including the dinosaurs, went extinct.

·        The Permian mass extinction happened at a time of enormous volcanic eruptions in what is now in Siberia.

°        These eruptions may have produced enough carbon dioxide to warm the global climate.

°        Reduced temperature differences between the equator and the poles would have slowed the mixing of ocean water.

°        The resulting oxygen deficit in the oceans may have played a large role in the Permian extinction.

·        A clue to the Cretaceous mass extinction is a thin layer of clay enriched in iridium that separates sediments from the Mesozoic and Cenozoic.

°        Iridium is a very rare element on Earth that is common in meteorites and other objects that fall to Earth.

°        Walter and Luis Alvarez and their colleagues at the University of California proposed that this clay is fallout from a huge cloud of debris that was thrown into the atmosphere when an asteroid or a large comet collided with Earth.

§        The cloud would have blocked sunlight and disrupted the global climate for several months.

°        A 65-million-year-old crater scar has been located beneath sediments on the Yucatán coast of Mexico.

§        At 180 km in diameter, it is the right size to have been caused by an object with a diameter of 10 km.

·        Much remains to be learned about the causes of mass extinctions.

°        It is clear that they provided life with opportunities for adaptive radiations into newly vacated ecological niches.

 

Concept 26.3 As prokaryotes evolved, they exploited and changed young Earth

·        The oldest known fossils are 3.5-billion-year-old stromatolites, rocklike structures composed of layers of cyanobacteria and sediment.

·        If bacterial communities existed 3.5 billion years ago, it seems reasonable that life originated much earlier, perhaps 3.9 billion years ago, when Earth first cooled to a temperature where liquid water could exist.

 Prokaryotes dominated evolutionary history from 3.5 to 2.0 billion years ago.

·        The early protobionts must have used molecules present in the primitive soup for their growth and replication.

·        Eventually, organisms that could produce all their needed compounds from molecules in their environment replaced these protobionts.

°        A rich variety of autotrophs emerged, some of which could use light energy.

·        The diversification of autotrophs allowed the emergence of heterotrophs, which could live on molecules produced by the autotrophs.

·        Prokaryotes were Earth’s sole inhabitants from 3.5 to 2.0 billion years ago.

°        These organisms transformed the biosphere of the planet.

·        Relatively early, prokaryotes diverged into two main evolutionary branches, the bacteria and the archaea.

°        Representatives from both groups thrive in various environments today.

 Metabolism evolved in prokaryotes.

·        The chemiosmotic mechanism of ATP synthesis is common to all three domains—Bacteria, Archaea, and Eukarya.

°        This is evidence of a relatively early origin of chemiosmosis.

·        Transmembrane proton pumps may have functioned originally to expel H⁺ that accumulated when fermentation produced organic acids as waste products.

°        The cell would have to spend a large portion of its ATP to regulate internal pH by driving H⁺ pumps.

°        The first electron transport pumps may have coupled the oxidation of organic acids to the transport of H⁺ out of the cell.

·        Finally, in some prokaryotes, electron transport systems efficient enough to expel more H⁺ than necessary to regulate pH evolved.

·        These cells could use the inward gradient of H⁺ to reverse the H⁺ pump, which now generated ATP instead of consuming it.

°        Such anaerobic respiration persists in some present-day prokaryotes.

·        Photosynthesis probably evolved very early in prokaryotic history.

°        The metabolism of early versions of photosynthesis did not split water and liberate oxygen.

°        Some living prokaryotes display such nonoxygenic photosynthesis.

·        The only living photosynthetic prokaryotes that generate O₂ are cyanobacteria.

·        Most atmospheric oxygen is of biological origin, from the water-splitting step of photosynthesis.

°        When oxygenic photosynthesis first evolved, the free oxygen it produced likely dissolved in the surrounding water until the seas and lakes became saturated with O₂.

°        Additional O₂ then reacted with dissolved iron to form the precipitate iron oxide.

°        These marine sediments were the source of banded iron formations, red layers of rock containing iron oxide that are a valuable source of iron ore today.

°        About 2.7 billion years ago, oxygen began accumulating in the atmosphere and terrestrial rocks with oxidized iron formed.

·        While oxygen accumulation was gradual between 2.7 and 2.2 billion years ago, it shot up to 10% of current values shortly afterward.

·        This oxygen revolution had an enormous impact on life.

·        In its free molecular and ionized forms and in compounds such as hydrogen peroxide, oxygen attacks chemical bonds, inhibits enzymes, and damages cells.

°        The increase in atmospheric oxygen likely doomed many prokaryote groups.

°        Some species survived in habitats that remained anaerobic, where their descendents survive as obligate anaerobes.

·        Other species evolved mechanisms to use O₂ in cellular respiration, which uses oxygen to help harvest the energy stored in organic molecules.

 

Concept 26.4 Eukaryotic cells arose from symbioses and genetic exchanges between prokaryotes

·        Eukaryotic cells differ in many respects from the smaller cells of bacteria and archaea.

°        Even the simplest single-celled eukaryote is far more complex in structure than any prokaryote.

·        While there is some evidence of earlier eukaryotic fossils, the first clearly identified eukaryote appeared about 2.1 billion years ago.

°        Other fossils that resemble simple, single-celled algae are slightly older (2.2 billion years) but may not be eukaryotic.

°        Traces of molecules similar to cholesterol are found in rocks dating back 2.7 billion years.

§        Such molecules are found only by aerobically respiring eukaryotic cells.

§        If confirmed, this would place the earliest eukaryotes at the same time as the oxygen revolution that changed the Earth’s environment so dramatically.

·        Prokaryotes lack internal structures such as the nuclear envelope, endoplasmic reticulum, and Golgi apparatus.

°        They have no cytoskeleton and are unable to change cell shape.

·        Eukaryotic cells have a cytoskeleton and can change shape, enabling them to surround and engulf other cells.

°        The first eukaryotes may have been predators of other cells.

·        A cytoskeleton enables a eukaryotic cell to move structures within the cell and facilitates the movement of chromosomes in meiosis and mitosis.

°        Mitosis made it possible to reproduce the large eukaryotic genome.

°        Meiosis allowed sexual recombination of genes.

·        How did the complex organization of the eukaryotic cell evolve from the simpler prokaryotic condition?

°        A process called endosymbiosis probably led to mitochondria and plastids (the general term for chloroplasts and related organelles).

·        The endosymbiotic theory suggests that mitochondria and plastids were formerly small prokaryotes living within larger cells.

°        The term endosymbiont is used for a cell that lives within a host cell.

·        The proposed ancestors of mitochondria were aerobic heterotrophic prokaryotes.

·        The proposed ancestors of plastids were photosynthetic prokaryotes.

·        The prokaryotic ancestors of mitochondria and plastids probably gained entry to the host cell as undigested prey or internal parasites.

·        The symbiosis became mutually beneficial.

°        A heterotrophic host could use nutrients released from photosynthesis.

°        An anaerobic host would have benefited from an aerobic endosymbiont.

·        As they became increasingly interdependent, the host and endosymbionts became a single organism.

·        All eukaryotes have mitochondria or their genetic remnants.

°        The theory of serial endosymbiosis supposes that mitochondria evolved before plastids.

·        Overwhelming evidence supports an endosymbiotic origin of plastids and mitochondria.

°        The inner membranes of both organelles have enzymes and transport systems that are homologous to those in the plasma membranes of modern prokaryotes.

°        Mitochondria and plastids replicate by a splitting process similar to prokaryotic binary fission.

°        Like prokaryotes, each organelle has a single, circular DNA molecule that is not associated with histone.

°        These organelles contain tRNAs, ribosomes, and other molecules needed to transcribe and translate their DNA into protein.

°        Ribosomes of mitochondria and plastids are similar to prokaryotic ribosomes in terms of size, nucleotide sequence, and sensitivity to antibiotics.

·        Which prokaryotic lineages gave rise to mitochondria and plastids?

°        Comparisons of small-subunit ribosomal RNA from mitochondria, plastids, and various living prokaryotes suggest that a group of bacteria called the alpha proteobacteria are the closest relatives to mitochondria and that cyanobacteria are the closest relatives to plastids.

·        Over time, genes have been transferred from mitochondria and plastids to the nucleus.

·        This process may have been accomplished by transposable elements.

°        Some mitochondrial and plastic proteins are encoded by the organelle’s DNA, while others are encoded by nuclear genes.

°        Some proteins are combinations of polypeptides encoded by genes in both locations.

·        The origins of other aspects of eukaryotic cells are unclear.

°        Some researchers have proposed that the nucleus itself evolved from an endosymbiont.

°        Nuclear genes with close relatives in both bacteria and archaea have been found.

·        The genome of eukaryotic cells may be the product of genetic annealing, in which horizontal gene transfers occurred between many different bacterial and archaeal lineages.

°        These transfers may have taken place during the early evolution of life, or may have happened repeatedly until the present day.

·        The origin of other eukaryotic structures is also the subject of active research.

°        The Golgi apparatus and the endoplasmic reticulum may have originated from infoldings of the plasma membrane.

°        The cytoskeletal proteins actin and tubulin have been found in bacteria, where they are involved in pinching off bacterial cells during cell division.

°        These bacterial proteins may provide information about the origin of the eukaryotic cytoskeleton.

·        Some investigators have suggested that eukaryotic flagella and cilia evolved from symbiotic bacteria.

°        However, the 9+2 microtubule apparatus of eukaryotic flagella and cilia has not been found in any prokaryotes.

 

Concept 26.5 Multicellularity evolved several times in eukaryotes

·        A great range of eukaryotic unicellular forms evolved as the diversity of present-day “protists.”

·        Molecular clocks suggest that the common ancestor of multicellular eukaryotes lived 1.5 billion years ago.

°        The oldest known fossils of multicellular eukaryotes are 1.2 billion years old.

°        Recent fossil finds from China have produced a diversity of algae and animals from 570 million years ago, including beautifully preserved embryos.

·        Why were multicellular eukaryotes so limited in size, diversity, and distribution until the late Proterozoic?

·        Geologic evidence suggests that a severe ice age gripped Earth from 750 to 570 million years ago.

°        According to the snowball Earth hypothesis, life would have been confined to deep-sea vents and hot springs or those few locations where enough ice melted for sunlight to penetrate the surface waters of the sea.

°        The first major diversification of multicellular eukaryotic organisms corresponds to the time of the thawing of snowball Earth.

·        The first multicellular organisms were colonies.

°        Some cells in the colonies became specialized for different functions.

°        Such specialization can be seen in some prokaryotes.

°        For example, certain cells of the filamentous cyanobacterium Nostoc differentiate into nitrogen-fixing cells called heterocysts, which cannot replicate.

·        The evolution of colonies with cellular specialization was carried much further in eukaryotes.

°        A multicellular eukaryote generally develops from a single cell, usually a zygote.

°        Cell division and cell differentiation help transform the single cell into a multicellular organism with many types of specialized cells.

°        With increasing cell specialization, specific groups of cells specialized in obtaining nutrients, sensing the environment, etc.

°        This division of function eventually led to the evolution of tissues, organs, and organ systems.

·        Multicellularity evolved several times among early eukaryotes.

 Animal diversity exploded during the early Cambrian period.

·        Most of the major phyla of animals appear suddenly in the fossil record in the adaptive radiation known as the Cambrian explosion.

·        Cnidarians (the phylum that includes jellies) and poriferans (sponges) were already present in the late Precambrian.

·        However, most of the major groups (phyla) of animals make their first fossil appearances during the relatively short span of the Cambrian period’s first 20 million years.

·        Molecular evidence suggests that animal phyla originated and began to diverge between 1 billion and 700 million years ago.

·        At the beginning of the Cambrian, these phyla suddenly and simultaneously increased in diversity and size.

 Plants, fungi, and animals colonized the land about 500 million years ago.

·        The colonization of land was one of the pivotal milestones in the history of life.

°        There is fossil evidence that cyanobacteria and other photosynthetic prokaryotes coated damp terrestrial surfaces well more than a billion years ago.

°        However, macroscopic life in the form of plants, fungi, and animals did not colonize land until about 500 million years ago, during the early Paleozoic era.

·        The gradual evolution from aquatic to terrestrial habitats was associated with adaptations that allowed organisms to prevent dehydration and to reproduce on land.

°        For example, plants evolved a waterproof coating of wax on their photosynthetic surfaces to slow the loss of water.

·        Plants colonized land in association with fungi.

°        In the modern world, the roots of most plants are associated with fungi that aid in the absorption of water and nutrients from the soil.

§        The fungi obtain organic nutrients from the plant.

°        This ancient symbiotic association is evident in some of the oldest fossilized roots.

·        Plants created new opportunities for all life, including herbivorous (plant-eating) animals and their predators.

·        The most widespread and diverse terrestrial animals are arthropods (including insects and spiders) and vertebrates (including amphibians, reptiles, birds, and mammals).

°        Terrestrial vertebrates, which include humans, are called tetrapods because of their four limbs.

 Earth’s continents drift across the planet’s surface on great plates of crust.

·        Earth’s continents drift across the planet’s surface on great plates of crust that float on the hot, underlying mantle.

°        Plates may slide along the boundary of other plates, pulling apart or pushing against each other.

·        Continental plates move slowly, but their cumulative effects are dramatic.

°        Mountains and islands are built at plate boundaries or at weak points on the plates.

·        Plate movements have had a major influence on life.

°        About 250 million years ago, near the end of the Paleozoic era, all the continental landmasses came together into a supercontinent called Pangaea.

°        Ocean basins deepened, sea level lowered, and shallow coastal seas drained.

§        Many marine species living in shallow waters were driven extinct by the loss of habitat.

°        The interior of the supercontinent was severe, cold, and dry, leading to much terrestrial extinction.

°        During the Mesozoic era, 180 million years ago, Pangaea began to break up.

§        As the continents drifted apart, each became a separate evolutionary arena with lineages of plants and animals that diverged from those on other continents.

·        Continental drift explains much about the former and current distribution of organisms.

°        Australian flora and fauna contrast sharply from that of the rest of the world.

§        Marsupial mammals fill ecological roles in Australia analogous to those filled by placental mammals on other continents.

°        Marsupials probably evolved first in what is now North America and reached Australia via South America and Antarctica while the continents were still joined.

°        The breakup of the southern continents set Australia adrift.

§        In Australia, marsupials diversified and the few early eutherians became extinct.

§        On other continents, marsupials became extinct and eutherians diversified.

 

Concept 26.6 New information has revised our understanding of the tree of life

·        In recent decades, molecular data have provided new insights into the evolutionary relationships of life’s diverse forms.

·        The first taxonomic schemes divided organisms into plant and animal kingdoms.

·        In 1969, R. H. Whittaker argued for a five-kingdom system: Monera, Protista, Plantae, Fungi, and Animalia.

°        The five-kingdom system recognized that there are two fundamentally different types of cells: prokaryotic (the kingdom Monera) and eukaryotic (the other four kingdoms).

·        Three kingdoms of multicellular eukaryotes were distinguished by nutrition, in part.

°        Plants are autotrophic, making organic food by photosynthesis.

°        Most fungi are decomposers with extracellular digestion and absorptive nutrition.

°        Most animals ingest food and digest it within specialized cavities.

·        In Whittaker’s system, Protista included all eukaryotes that did not fit the definition of plants, fungi, or animals.

°        Most protists are unicellular.

°        However, some multicellular organisms, such as seaweeds, were included in Protista because of their relationships to specific unicellular protists.

°        The five-kingdom system prevailed in biology for more than 20 years.

·        During the past three decades, systematists applied cladistic analysis to taxonomy, constructing cladograms based on molecular data.

°        These data led to the three-domain system of Bacteria, Archaea, and Eukarya as “superkingdoms.”

°        Bacteria differ from Archaea in many key structural, biochemical, and physiological characteristics.

·        Many microbiologists have divided the two prokaryotic domains into multiple kingdoms based on cladistic analysis of molecular data.

·        A second challenge to the five-kingdom system comes from systematists who are sorting out the phylogeny of the former members of the kingdom Protista.

°        Molecular systematics and cladistics have shown that the Protista is not monophyletic.

°        Some of these organisms have been split among five or more new kingdoms.

°        Others have been assigned to the Plantae, Fungi, or Animalia.

·        Clearly, taxonomy at the highest level is a work in progress.

·        There will be much more research before there is anything close to a new consensus for how the three domains of life are related and how many kingdoms should be included in each domain.

°        New data, including the discovery of new groups, will lead to further taxonomic remodeling.

°        Keep in mind that phylogenetic trees and taxonomic groupings are hypotheses that fit the best available data.