Multicellular Life

Biologists estimate that 99% of the species which have ever lived on Earth are now extinct, and up to 80 million species populate our world today. It is the great diversity of species that allows at least some organisms to survive major changes in the environment.

4 billion years of simple, prokaryotic cells
3 billion years of photosynthesis
2 billion years of complex, eukaryotic (but still single!) cells
1 billion years of multicellular life

The history of life reaches the last billion years of Earth’s 4.6 billion-year history with no hint of the wondrous diversity of life as humans know it (Figure 1). Not until nearly 80% of Earth’s history had passed did multicellular life evolve. The fossil record tells the story: millions of species of fish, amphibians, reptiles, birds, mammals, mosses, ferns, conifers, flowering plants, and fungi populated the seas and covered the Earth - as continents crashed together and broke apart, glaciers advanced and retreated, and meteors struck, causing massive extinctions. Life has had a colorful and exciting last billion years, spawning diversity almost beyond our comprehension.

Figure 1: History of Earth.

And yet, the giant steps of evolution remain back in the Precambrian. Its catalog of evolutionary innovations is long and impressive:

• Energized elements from stardust formed simple organic molecules.
• Building blocks chained together to form catalysts and self-replicating macromolecules.
• Biochemical pathways evolved.
• Protective yet permeable membranes enclosed the catalysts, replicators and their metabolic retinue.
• Early prokaryotic cells “learned” to make ATP by splitting glucose.
• Others began to harvest sunlight energy through photosynthesis.
• Photosynthetic cyanobacteria produced vast amounts of “waste” oxygen, dramatically altering the Earth’s atmosphere.
• The oceans rusted (iron ore deposits).
• An ozone layer formed, shielding life from UV radiation.
• The “\(O_2\) catastrophe” killed many anaerobic prokaryotes.
• Still other prokaryotes “learned” to use the new O2 to release the energy remaining in carbohydrates products of glycolysis.
• Endosymbiosis created eukaryotes, firmly establishing the three major evolutionary lineages, which yet today comprise the living world.

The timing and exact nature of most of these innovations is speculative; indeed, the first few may have been extraterrestrial and even deeper in time. They comprise perhaps the most important landmarks in the evolution of life, but the fossil record is sketchy due to prokaryote size, rock layer metamorphosis, and burial by more recent rocks.

Overall, we know remarkably little about Precambrian life. The Cambrian Period documents the greatest flowering of life of all time, and gives its name - in a rather negative sense - to the 4 billion years of Earth history that preceded it. Before we dive into the famous Cambrian “explosion,” we will look more carefully at the last Eon of the Precambrian, which set the stage for this most famous burst of evolution.

Late Precambrian: Setting the Stage for an Explosion of Biodiversity

The geologic record of the Proterozoic, the most recent eon of the Precambrian, is much better than that of the Archean and Hadean Eons before it. Accordingly, we know that supercontinents formed by collision and broke apart by rifting. The atmosphere changed dramatically with the addition of oxygen and a protective ozone layer. Glaciations covered much of the Earth with ice so extensively that it is known as the “Snowball Earth” during that period (Figure 2). Eventually, enough \(CO_2\) escaped from volcanoes to begin a period of global warming; melting opened a great variety of new niches. The severe restriction and subsequent opening of opportunities may have driven the later Cambrian explosion.

Figure 2: The geologic record documents at least two ice ages during the last eon of the Precambrian. One was so severe that some scientists believe ice then covered the entire globe, and they dub it the “Snowball Earth.” The icy constriction of life and later meltdown opening of niches may have contributed to the explosive evolution of the Ediacaran and Cambrian Periods that followed.

Within this dramatic environmental panorama, the three major lineages of life – Bacteria, Archaea, and Eukaryotes continued to diversify. Plant, animal, and fungal ancestors diverged as solitary cells. Gradually, some of these cells began to live in colonies. Within the colonies, primitive specialization among cells made certain tasks more efficient. The modern green alga, Volvox illustrates a comparable level of organization (Figure 3). The line between colonies and multicellular organisms is difficult to draw, but most scientists agree that true plants had evolved by about 1 billion years ago, and animals evolved about 100 million years later.

Figure 3: The green alga Volvox shows the multicellularity and early cell specialization which probably characterized early colonial eukaryotes. Specializations include anterior sensory cells, asexual and two types of sexual reproductive cells, and coordination among flagellate cells.

The fossil record shows that some eukaryotes had begun to reproduce sexually by a little over a billion years ago (Figure 4). Sexual reproduction was a major evolutionary innovation, producing more variety among offspring and thus more rapid adaptation to changing environments.

Figure 4: The evolution of sexual reproduction around 1 billion years ago increased variety among offspring, and may have increased rates of evolution.

Near the end of the Precambrian - not until just over 600 million years ago, a unique assemblage of multicellular organisms left a fossil record which gives us our first glimpse of multicellular diversity – the Ediacaran biota (the name is taken from the hills in Australia where the first such fossils were found) (Figure 5).

Figure 5: Spriggina (top), an Ediacaran fossil, may be an ancestor of the trilobites. Charnia (bottom), the first accepted complex Precambrian organism, is more typical of the Ediacaran biota – it is difficult to show relationships to any modern species.

Members of this community include:

• some familiar organisms such as sponges, red and green algae, and bacteria
• very few ancestors of modern animals
• many unique disk, bag, and quilt like animals which do not resemble any modern animals

The origin and relatively rapid extinction of this entire group remain somewhat of a mystery. The oxygen atmosphere and/or an ice age may explain their initial radiation. Their abrupt and nearly complete disappearance may have resulted from unbalanced predation, grazing, or competition, or yet another environmental crisis such as supercontinent breakup, changes in ocean chemistry, and/or rising sea levels. Whatever the causes, most species disappeared by the end of the Precambrian, about 542 million years ago. The Ediacarans appear to have been an early multicellular, dead-end branch on the bush of life. Their extinction, however, appears to have paved the way for a spectacular evolution of much more familiar life, which marks the beginning of the modern Phanerozoic Eon: the Cambrian explosion.

Paleozoic Era: Ancient Plants and Animals, but Seeds of Modern Life

Figure 6: Paleozoic Era

The Paleozoic era of the current, Phanerozoic Eon is the first concrete chapter of life’s history (Figure 6). Abundant fossils, clearly related to modern animals, plants and fungi, illuminate the path of evolution beginning with its first Period, the Cambrian, 542 million years ago. However, the sudden appearance of such variety presents yet another puzzle in the story of life: how did roughly 50 major groups of organisms evolve so rapidly, without apparent ancestors? The abrupt emergence of so many phyla has given this period in geologic time its nickname, the Cambrian explosion, but its causes remain hypothetical. As for the Ediacaran radiation, major environmental changes have been proposed but not convincingly documented. A major geologic event of the Paleozoic is the amalgamation of the supercontinent Gondwana, but it does not seem to explain the extent of the increase in Cambrian diversity. Perhaps life itself was responsible: a “critical mass” of development could have opened up new body pattern options, or more kinds of life opened more kinds of ecological niches. Whatever the cause, the evidence shows that nearly all modern animal phyla, including our own chordate phylum, are represented in this diversity of life. Among the most common and famous are reef-building sponges and arthropods, known as trilobites (Figure 7). Both were diverse and abundant during the Cambrian but later became extinct. However, the phyla they represent persist today.

Figure 7: Two representatives of more than fifty modern animal phyla from the Cambrian explosion are reef-building sponges (left) and early arthropods known as trilobites (right). Both were abundant during the Cambrian and later became extinct; however, the phyla they represent persist to this day.

A major extinction marks the boundary between the Cambrian and Ordovician Periods 488 million years ago (Figure 8). In warm, shallow continental seas, Ordovician life rebounded:

• A great diversity of new invertebrates swam the seas.
• Liverworts may have been the first green plants to appear on land (Figure 9). • The first fish, jawless and bony-plated ostracoderms, swam slowly along shallow sea bottoms.

Figure 8: An artist’s rendition shows that the second period of the Paleozoic, the Ordovician, heralded a great diversity of invertebrates, including nautiloids, crinoids, and bivalves.

Figure 9: Among the first true plants, liverworts colonized the land during the Ordovician. Without vascular tissue, they were small and grew flat and low to the ground (right). Like all plants and nearly all eukaryotes, they had adopted sexual reproduction (left, female reproductive organ). Both photos are greatly magnified.

About 444 million years ago, a sharp drop in atmospheric \(CO_2\) led to glaciation and ended the long stable period of warm seas. The Ice Age affected marine genera severely; up to 60% disappeared! This major extinction marks the end of the Ordovician and the beginning of the Silurian Period.

During the Silurian, the glaciers retreated. Melting icecaps raised sea level, yet a new supercontinent, Euramerica, formed near the equator. In a long, stable greenhouse phase, warm shallow seas covered extensive equatorial landmasses, opening tropical habitats on land and in water:

• Reef-building corals and sea-scorpions evolved.
• The first jawed fishes joined armored jawless fishes and many invertebrates.
• Vascular plants solved the problem of carrying water into the air.
• Arthropods such as millipedes followed the plants onto land.

The Silurian ended about 416 million years ago with a minor extinction, which may have been due to an asteroid impact or increasing glaciation.

During the Devonian Period, terrestrial life expanded to include forests of clubmosses, horsetails, ferns, and the earliest seed-bearing plants and trees (Figure 10).

Figure 10: Devonian fish (above, left) evolved lobes which eventually allowed vertebrates to move to land. On land (below), clubmosses, horsetails, and ferns joined primitive seed plants and early trees to form the first forests. Seeds (above, right) allowed reproduction on dry land.

• Seeds allowed plants reproduce on dry land in the same way that shelled eggs would later help animals. Insects appeared, although they were wingless at first. • Squid-like animals and ammonite mollusks became abundant. • Lobe-like fins allowed some fish to lift their heads above water and breathe air in oxygen-poor waters.

About 360 million years ago, extinction struck over 20% of marine families and over 50% of all genera, ending the Devonian. One hypothesis suggests that the greening of the continents absorbed \(CO_2\) from the atmosphere, reducing the greenhouse effect and lowering temperatures.

Extensive coal deposits, fuel for our Industrial Revolution, characterize rocks of the Carboniferous Period which followed. Coal developed from new bark-bearing trees in widespread lowland swamps and forests. Fallen trees were buried without decaying – perhaps because animals and bacteria had not yet evolved digestive enzymes that could break down the new molecule, lignin, in the wood. Burial of carbon lead to a corresponding buildup of oxygen in the atmosphere; \(O_2\) at the time was an all-time high of 35% (compared to 21% today). Abundant oxygen probably encouraged evolution, especially on land.

Figure 11: Vertebrates moved to land during the Carboniferous, and amphibians became abundant. Early lizards (A) were able to move to drier land in part because their new, shelled egg (B) did not dry out. Trees in widespread swamps evolved bark (C) containing as-yet non-biodegradable lignin (D), leading to the eventual formation of the coal which fueled our Industrial Revolution. With the highest known levels of O2, giant insects such as dragonflies (E) flew the skies.

As illustrated in Figure 11:

• Giant insects took to the air.
• Vertebrates moved to land; amphibians were far larger and more abundant and diverse than today.
• The shelled egg allowed early reptiles to reproduce on land without drying out the embryo.
• Early gymnosperms, reproducing with pollen rather than sperm, colonized dry land.

Toward the end of the Carboniferous, the climate cooled. Glaciation and extinction mark the border between the Carboniferous and the last period of the Paleozoic Era, about 300 million years ago.

The Permian is best known for the dramatic event which ended not only the period but also the entire Paleozoic Era – an extinction of 95% of the then-living world. If we look more closely at the effects of continental geography on climate, perhaps we can begin to understand not only that massive extinction, but also the major events in evolution which preceded it. During the Permian, all the major landmasses of earth combined into a single supercontinent, known as Pangaea (Figure 12). As for today’s continents, much of the interior would have been dry with seasons of temperature change, because the oceans’ moderating effects were too distant. Pangaea’s size may have exaggerated this continental climate of seasons and drought. Three major groups of animals and plants evolved in response to Pangaea’s extensive arid niches.

Figure 12: The supercontinent Pangaea encompassed all of today’s continents in a single land mass. This configuration limited shallow coastal areas which harbor marine species, and may have contributed to the dramatic event which ended the Permian - the most massive extinction ever recorded.

• Reptiles, with claws, scaly skin, and shelled eggs, diversified, foreshadowing Mesozoic dinosaurs.
• Cycads and other gymnosperms, with cuticle-covered leaves to limit water loss and cones to bear seeds, dominated forests.
• Insects evolved entire life cycles on dry land; beetles and flies navigated land and air.

At the end of the Permian, an estimated 99.5% of individual organisms perished. Several factors may have contributed, and one factor relates again to Pangaea. Marine biodiversity is greatest in shallow coastal areas. A single continent has a much smaller shoreline than multiple continents of the same size. Perhaps this restriction of marine habitats contributed to the drastic loss of species, for up to 95% of marine species perished, compared to “only” 70% of land species. Another factor might have been massive basalt flow attributed to the time, which could have increased \(CO_2\) levels to precipitate global warming. Some scientists invoke extraterrestrial causes: a huge meteorite crater discovered in 2006 in Antarctica and dated to between 100 and 500 million years ago could represent an impact which darkened skies, decreased sunlight, and shut down photosynthesis. Although the cause remains unknown, fossils clearly document the fact of Earth’s most devastating extinction. The event closed the Paleozoic Era, and inevitably opened the door to a new burst of life in the Mesozoic.

Mesozoic Era: Age of the Dinosaurs

Figure 13: Mesozoic Era.

Following the “great dying” at the end of the Permian, a resurgence of evolution in the Mesozoic established the basis of modern life (Figure 13). The continents, which began as one, broke apart and eventually shifted into their present configuration. Rifting encouraged speciation (Figure 14). Relatively stable warm temperatures contributed once again to great diversification among animals.

Figure 14: A major geological change in the Mesozoic was the breakup of the supercontinent Pangaea into Laurasia and Gondwana, and eventually into the continents we know today. The breakup created new niches, contributing to speciation.

During the Triassic, early dinosaurs appeared on land as the archosaurs, in the ocean as ichthyosaurs, and in the air as pterosaurs (Figure 15). One line of reptiles gave rise to the first mammals and others to the earliest turtles and crocodiles. Seed ferns and conifers dominated the forests. Modern corals and fishes, and many modern insects, evolved. The Triassic gave way to the Jurassic with one of the most active periods of volcanism ever recorded. Pangaea began to break apart. The major extinction marking the border between these two Periods opened niches which made way for the Age of the Dinosaurs.

Figure 15: Early dinosaurs branched off from other reptiles in the Triassic. The dinosaurs radiated into diverse niches – many undoubtedly newly opened by the massive Permian extinction. Pterosaurs (A) inhabited the air, archosaurs (B) the land, and ichthyosaurs (C) the seas. Not all dinosaurs were giant, as the size comparison of archosaurs to the average adult human (B, inset) shows.

The Jurassic Period was the golden age of the large dinosaurs which lived amidst warm, fern-and cycad-filled forests of pines, cedars, and yews (Figure 16). Dinosaurs included widespread and huge herbivorous sauropods, smaller predatory theropods, stegosaurs, and pterosaurs. Ichthyosaurs and plesiosaurs thrived in the oceans. Ammonites, sea urchins, and starfish were abundant invertebrates. The first birds and lizards appeared. One of the most famous transition fossils, Archaeopteryx, with characteristics of both reptiles and birds, dates from this Period (Figure 17). During the Jurassic, the supercontinent Pangaea broke apart into Laurasia and Gondwana.

Figure 16: The Jurassic was the golden age of large dinosaurs. Coniferous trees, also huge, and fern and cycad swamps formed their habitats.

Figure 17: One of the most famous of all transitional fossils is Archaeopteryx, “ancient wings.” The fossil dates back to the Jurassic. Both reptilian features (teeth and claws) and avian features (wings and feathers) are clear.

Flowering plants first appeared in the Jurassic, but dominated the last, Cretaceous Period of the Mesozoic.

• New kinds of insects coevolved with the flowering plants, serving as their pollinators.

An early example of this coevolution is the magnolia, which developed flowers to attract – and withstand feeding damage from - beetle pollinators. Bees first appeared during the Cretaceous, and figs evolved unusual flower-fruits in concert with tiny wasp pollinators (Figure 18).

Figure 18: Plants first evolved flowers during the Cretaceous. Flowers attracted and fed insects, and insects, in turn, pollinated the flowers, leading to a long coevolutionary relationship. Cretaceous examples include the magnolia and its beetle pollinators (left and below), and the unique fig “fruit”-flower and its tiny wasp pollinator (top right).

• Primitive birds arose from reptilian ancestors and soon out-competed many of the pterosaurs. • All three major groups of mammals – monotremes, marsupials, and placentals – became established, but remained small.

In part because a huge sea (the Tethys) formed an east-west connection between the oceans, Cretaceous climate was uniformly warm; even the poles lacked ice. In response, warmadapted plants and dinosaurs expanded to within 15 degrees of the poles. Dinosaurs reached a peak of diversity and size (Figure 19 and Figure 20).

Figure 19: Many kinds of reptiles and invertebrates lived during the Cretaceous Period. Mosasaurs (upper left), plesiosaurs (center) and ammonites (upper right) swam the seas with modern sharks. Triceratops (lower left) and duckbilled dinosaurs (lower right) show some of Cretaceous diversity in dinosaurs.

Figure 20: Moderate climate worldwide during the Cretaceous encouraged great size and diversification among dinosaurs. The herbivorous titanosaur, Argentinosaurus (above) may have been the largest of all the dinosaurs, weighing in at up to 100 tons. Gigantosaurus (below) probably preyed upon titanosaurs such as Argentinosaurus, but weighed “only” 5.2 tons, and despite a bath-tub-sized skull, operated on a brain the size of a banana.

• Titanosaurs, including possibly the largest of all the dinosaurs, the 100-ton Argentinosaurus, were the dominant herbivores. A single Argentinosaurus vertebra was 1.3 meters long, and its tibia would have been as tall as some humans. Fossilized eggs, containing embryos with skin, indicate that titanosaurs were colonial nesters. Fossilized dung shows they ate cycads and conifers, but also palms and the ancestors of rice and bamboo; some scientists suggest that dinosaurs and grasses coevolved like insects and flowering plants.

• One of the largest predatory dinosaurs, Giganotosaurus, weighed “only” 5.2 tons, but in length surpassed Tyrannosaurus rex by two meters (six feet). Giganotosaurus’ skull was the size of a bathtub, but its brain was the size and shape of a banana!

The dramatic extinction of all dinosaurs (except the lineage which led to birds) marked the end of the Cretaceous. Dinosaurs had begun to decline earlier, perhaps due to reduction in atmospheric oxygen and global cooling. A worldwide iridium-rich layer, dated at 65.5 million years ago, provides evidence for an additional, more dramatic cause for their ultimate extinction. Iridium is rare in the Earth’s crust, but common in comets and asteroids. Scientists correlate this layer with a huge crater in the Yucatan and Gulf of Mexico. A collision/ explosion between the Earth and a comet or asteroid could have spread debris which set off tsunamis, altered the climate (including acid rain), and reduced sunlight 10-20%. A consequent reduction in photosynthesis would have caused a drastic disruption in food chains. Some scientists believe that volcanism also contributed to the “K-T” (Cretaceous-Tertiary) extinction, but most agree that “an impact event” was at least a major cause (Figure 21). The massive extinction and sharp geologic line led geologists to define the end of the Mesozoic and the beginning of our modern Era, the Cenozoic, with this event.

Figure 21: The extinction of the dinosaurs at the end of the Cretaceous is attributed at least in part to an impact event which could have involved a meteor, an asteroid, or a comet.

Cenozoic Era: Age of Modern Life

Neogene and Quaternary (Q) Periods share part of the Pliocene Epoch (Pl). Pleistocene (P) and Holocene (H) Epochs complete the Quaternary Period. Divisions in this part of the Time Scale are debated and may change.

Figure 22: Cenozoic Era.

The Cenozoic Era brings the history of life into the present, but not without drama, mystery, and the looming possibility of a “Sixth Extinction” (Figure 22) You probably know the basic story: mammals took over where dinosaurs left off, branched to form primates, moved to the grasslands, became human-like, survived the ice ages, and the rest is – literally – history. Let’s look at some of the major events, focusing not only on our immediate ancestors but also on the world in which they evolved.

Seven Epochs comprise the Cenozoic Era, with the Holocene continuing up to today. “Tertiary” refers to the 64 million years and five epochs before the Quaternary Period, well known for its recent ice ages and recognizable humans. Tertiary and Quaternary periods could be called suberas, but current organization of the Cenozoic segment of the Geologic Time Scale is the subject of current debate; it may well change.

The Paleocene Epoch provided a worldwide warm, humid climate for the rapid evolution which followed the extinction of the dinosaurs (Figure 23). Many plants, herbivores, and carnivores had disappeared because they depended on photosynthesis, but omnivores, insectivores, and scavengers – which included many mammals and birds – survived because their food sources actually increased. Mammals radiated into the ecological niches opened up by the extinction of herbivores and carnivores, and larger species, up to bear- or hippopotamussized, began to appear In equatorial regions, the first recognizably modern rain forests appeared, and south of the equator, hot arid regions provided niches for new groups of plants, including cacti.

Figure 23: During the Paleocene, mammals and birds invaded ecological niches formerly occupied by the dinosaurs. Mammals included monotremes (A), marsupials, and hoofed placentals (B). Modern sharks (C) patrolled the seas. Birds included the giant flightless Gastornis (D).

Volcanism or a massive release of methane gas trapped in the oceans may have triggered one of the most rapid global warming events ever measured at the beginning of the Eocene, 56 million years ago. CO2 from either volcanism or oxidation of methane would have caused the oceans to become more acidic, and Earth’s temperatures to rise. Warm temperatures allowed forests of dawn redwood, swamp cypress, and palms to extend toward both poles. In the interiors of the continents, seasonal temperature and moisture variations led to the evolution of grasses, expansive savannas and deciduous forests. Within these new ecosystems, modern mammals with specialized teeth evolved. Probably due to high temperatures, these mammals were smaller than those who preceded them – or those who followed:

• Horses and tapirs evolved in North America, and rhinoceros evolved in Asia.
• Primates, with their long arms and legs and grasping hands and feet, appeared.
• Mammals returned to the sea; Basilosaurus was an ancestor of today’s whales.

At the beginning of the Eocene, Australia was still connected to Antarctica, but when they broke apart, ocean currents changed and cooling began in earnest, foreshadowing the ice ages to come. Tundra ecosystems developed near the poles. Falling sea levels, a land bridge immigration of mammals from Asia to North America, and perhaps several impact events led to an extinction which marks the end of this epoch.

Figure 24: The Oligocene produced fewer new mammals than the Eocene; most were adapted to grasslands. Mesohippus (A) showed small steps toward modern horses. Hyaenodon (B) had the large, sharp teeth of a carnivore. Elotherium (C) was a piglike scavenger, and Arsinoitherium (D) was a large relative of elephants and hyraxes. Perhaps the largest land mammal of all time resembled an overweight giraffe; Indricotherium (E) weighed up to 15 tons and reached 18 feet in height.

As its name implies, the Oligocene Epoch produced a “few” new mammals, especially in grasslands and savannahs (Figure 24).

• Pig-like entelodonts used massive skulls to crush bones of scavenged prey.
• One of the largest land mammals of all time, the 18-foot, 15 ton Indricotherium, ate leaves from the tops of trees in the manner of a giraffe.
• Horses, represented by Mesohippus remained small relative to today’s species.
• Large terrestrial carnivores such as Hyaenodon, hunted mammals up to the size of sheep.
• The rhinoceros-like Arsinoitherium wandered tropical rain forests and swamps.

By the beginning of the Miocene Epoch 23 million years ago, the continents had almost assumed their current configuration, except that North and South America did not connect. Oceans continued to cool, ice caps expanded at the poles, and consequently the climate dried. Grasslands, needing less rain, replaced forests, and large herbivores coevolved with the grasses. Modern mammals, including wolves, beaver, deer, camels, seals, dolphins, and porpoises, evolved. Up to 100 species of apes lived throughout Africa, Europe, and Asia. Almost all modern bird groups were represented.

The Earth’s climate continued to cool into the Pliocene, the epoch in which hominids first appeared. Seasons became more pronounced; deciduous forests and grasslands replaced tropical forests, and coniferous forests and tundra expanded. Large mammals, such as browsing mastodons and grazing mammoths, roamed the grasslands and tundra. Into this setting walked Australopithecines, such as Lucy who share common ancestry with humans. Fossil footprints dated as 3.7 million years old establish Australopithecenes as bipedal – perhaps the first apes to walk upright (Figure 25). Later Pliocene hominids included two members of our own genus, Homo rudolfensis and Homo habilis. During this epoch, falling sea levels exposed two land bridges which allowed important migrations.

Figure 25: Lucy (D) is one of the most complete fossils of Australopithecus afarensis, a human relative also known for fossil footprints which establish an upright posture. Note that the brown bones are Lucy’s; others have been added to restore her skeleton. Australopithecines coexisted (but not necessarily on the same continent!) with browsing mastodons (A), grazing mammoths (B), and giant herbivorous sloths (C).

• One allowed horses, mammoths, mastodons and more to migrate between Asia and North America. • A second allowed North American placental mammals, such as giant sloths, armadillos, and sabertooth cats, to migrate to South America. Placentals eventually out-competed all of South America’s marsupials except the opossum.

Repeated glaciations define the Pleistocene Epoch. Glaciation tied up huge volumes of water in ice packs; rainfall was less, because evaporation was less. Deserts were relatively dry. During interglacial periods, huge inland lakes and rivers held or carried the melt waters, and coastal flooding reduced land area. During the four major glaciations, these severe climate changes stressed animals and plants, encouraged the evolution of large animals (the Pleistocene megafauna), and forced life toward the equator.

Figure 26: That Homo erectus (A), and later (or in other parts of the world) Homo habilis, hunted mammoth (B) is shown by fossil evidence 1.8 million years old. Woolly mammoths (C), specially adapted to cold climate, were probably hunted, as well, as humans spread throughout the world. Nearly 40 woolly mammoth remains have been found preserved in permafrost, complete with soft tissue and DNA. To date, mitochondrial DNA has been sequenced. The calf (D) measures 2.3 meters (8 feet) long. A predatory competitor to humans was the saber-tooth tiger, (E).

• Some adapted to the cold: the Woolly Mammoth grew thick, shaggy hair oiled by abundant sebaceous glands, a layer of fat beneath the skin, smaller ears, and even a convenient flap to cover the anus, keeping out the cold. Mammoth teeth ground tough tundra grasses, and their long, curved tusks may have helped to clear snow. Permafrosts have preserved nearly 40 mammoth remains, including soft tissues, and scientists actually hope to be able to recreate its genome; mitochondrial DNA for one species has already been sequenced! Using this sequence as a molecular clock, scientists calculate that mammoths diverged from African elephants about 6 million years ago, roughly the same time that humans diverged from chimpanzees.

• Saber-tooth cats used dagger-like teeth to cut their prey’s windpipe and jugular veins, causing death by bleeding. Many saber-tooths have been found in the LaBrea Tar Pits in southern California, where they had tried to feed on mammoths trapped before them in the sticky tar/asphalt.

• Homo erectus, the dominant hominid during the Pleistocene, migrated throughout Africa, Europe, and Asia, giving rise to a number of variations of hominids. Although Homo erectus was probably the first hominid to leave Africa, the species may not have been a direct ancestor of humans. Pleistocene hominids were hunter-gatherers; evidence dated at 1.8 million years ago supports their consumption of mammoth.

A major extinction of Pleistocene megafauna continued into the Holocene. Some attribute the extinction to changing climate or disease, but others have connected the migrations of humans to each continent’s time of extinction The “overkill” theory suggests that humans hunted large animals with too much success. Agreement is not yet universal, but most scientists admit the evidence is strong.

The current Holocene Epoch began 11,550 years ago (about 9600 B.C.) with the retreat of the Pleistocene glaciers. During the Holocene, melting ice has raised sea level over 180 meters (600 feet). Geologists believe that we are currently experiencing an interglacial warming, and that glaciers will return – unless continued human burning of fossil fuels raises \(CO_2\) levels to bring about global warming. All of human civilization has occurred within the Holocene; Homo sapiens have passed through Mesolithic, Neolithic, and Bronze Age civilizations. We will examine the possibility that humans are currently causing a mass extinction which some compare to the Permian. Many would include the Pleistocene megafauna in this “Sixth Extinction,” citing the “overkill theory” data in Figure 27. Some even call the period of time from that loss to the present the “Anthropocene epoch” to describe the major impact humans have had on the planet and its life. Human population has surpassed 6.6 billion, and over-fishing, climate change, industrialization, intensive agriculture, and clearance of grasslands and rainforests contribute to a startlingly high loss of life.

Figure 27: This data comparing the arrival of humans to the decline of the Pleistocene megafauna supports the “overkill” theory that human predation contributed to the extinction of large mammals throughout the world. Other theories involve climate change and disease.

Paleontologists estimate that background extinction rates throughout most of life’s history averaged between 1 and 10 species per year (Figure 28) . The present rate of extinction is thought to be 100 to 1000 times ”background” rates, suggesting that the number of species which currently disappear each year could exceed 1,000! Biologist E.O. Wilson has predicted that current rates will result in the loss of over half of life’s biodiversity within the next one hundred years. In contrast, Earth’s shortest previous extinctions spanned several hundred thousand to several million years, and evidence for cause is entirely geological in nature. No other species has influenced the Earth and its life as powerfully as Homo sapiens.

Figure 28: (A) Changes in \(CO_2\) levels (green) are clearly associated with temperature changes (blue); the graph shows the four major Ice ages of the Pleistocene. Graphs B (long time scale) and C (recent time) show increases in \(CO_2\) and global temperature over the past 150 years, suggesting that the Industrial Revolution, which began our major fossil fuel burning and release of \(CO_2\), may account for much of the increase in temperature – a new, human–induced global warming.

Others say there is ample evidence to show that extinction is a natural phenomenon which has occurred repeatedly throughout the history of life on Earth. They point to the recoveries – indeed, radiations – which filled vacated ecological niches after each event.

However, those who are concerned about the current extinction wonder whether or not humans will be one of the species to become extinct. Because we are the only surviving members of our family, recovery or radiation would not be an option.