CK-12 Biology I - Honors
Page 49
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 below). Eventually, enough CO2 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 11.27
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 below). 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 11.28
The green alga 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 below). Sexual reproduction was a major evolutionary innovation, producing more variety among offspring and thus more rapid adaptation to changing environments.
Figure 11.29
The evolution of sexual reproduction around 1 billion years ago increased variety among offspring, and may have increased rates of evolution (see chapter).
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 below).
Figure 11.30
(top), an Ediacaran fossil, may be an ancestor of the trilobites. (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 11.31
The Paleozoic era of the current, Phanerozoic Eon is the first concrete chapter of life’s history (Figure above). 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 below). Both were diverse and abundant during the Cambrian but later became extinct. However, the phyla they represent persist today.
Figure 11.32
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 below). In warm, shallow continental seas, Ordovician life rebounded:
Figure 11.33
An artists rendition shows that the second period of the Paleozoic, the Ordovician, heralded a great diversity of invertebrates, including nautiloids, crinoids, and bivalves.
A great diversity of new invertebrates swam the seas.
Liverworts may have been the first green plants to appear on land (Figure below).
The first fish, jawless and bony-plated ostracoderms, swam slowly along shallow sea bottoms.
Figure 11.34
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 CO2 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. I
n 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 below).
Figure 11.35
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 CO2 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; O2 at the time was an all-time high of 35% (compared to 21% today). Abundant oxygen probably encouraged evolution, especially on land.
Figure 11.36
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 O, giant insects such as dragonflies (E) flew the skies.
As illustrated in Figure above:
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 below). 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 11.37
The supercontinent Pangaea encompassed all of todays 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 CO2 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 11.38
Following the “great dying” at the end of the Permian, a resurgence of evolution in the Mesozoic established the basis of modern life (Figure above). The continents, which began as one, broke apart and eventually shifted into their present configuration. Rifting encouraged speciation (Figure below). Relatively stable warm temperatures contributed once again to great diversification among animals.
Figure 11.39
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 below). 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 11.40
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 below). 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 below). During the Jurassic, the supercontinent Pangaea broke apart into Laurasia and Gondwana.
Figure 11.41
The Jurassic was the golden age of large dinosaurs. Coniferous trees, also huge, and fern and cycad swamps formed their habitats.
Figure 11.42
One of the most famous of all transitional fossils is 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.
Figure 11.43
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).
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 above).
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.