The History of Life on Earth • Defining Biological Evolution • Determining Earth’s Age • The Changing Face of Earth • The Fossil Record • Major Patterns in the History of Life on Earth • Rates of Evolutionary Change within Lineages • The Future of Evolution
Defining Biological Evolution • Understanding evolution is important because the features of all organisms are best understood in the light of evolution. • It is also important because humans are becoming powerful agents of evolutionary change.
Defining Biological Evolution • Biological evolution is a change over time in the genetic composition of a population of organisms. • Some changes can occur rapidly enough to be manipulated experimentally; others take place over very long time frames. • An understanding of the long-term patterns of evolutionary change requires thinking in time frames spanning many millions of years and imagining conditions on Earth that are very different from those we observe today.
Determining Earth’s Age • Determining the actual age of rocks is difficult. Determining the ages of rocks relative to one another is easier. • Geologists use the observation that in undisturbed strata (layers), young rocks are found on top of older rocks. • Fossils are remains of ancient organisms contained within rocks. • In general, fossils of similar ages are found in similar strata across the earth.
Determining Earth’s Age • Radioactivity provides a way to date rocks. • Radioactive isotopes decay in a predictable pattern over long periods of time. • The time it takes for half of a radioactive isotope to decay is that isotope’s half-life. • Each radioisotope has a characteristic half-life.
Determining Earth’s Age • To use a radioisotope to date a past event, the concentration of the isotope at the time of that event must be known or estimated. • In the case of 14C, we know that the ratio of 14C to 12C is relatively constant in the environment and living organisms. When an organism dies, 14C is no longer taken up by the cells, and the ration of 14C to 12C decreases through time. • 14C can be used to date fossils (and sedimentary rocks they were deposited in) less than 50,000 years old.
Determining Earth’s Age • Sedimentary rocks are unreliable for dating. • To date sedimentary rocks, geologists look for lava flows between sedimentary layers. The lava can be dated by the decay of potassium-40 to argon-40. • When radioactive dating methods are not applicable, alternative approaches and observations are used, including paleomagnetism, continental drift, sea level changes, and molecular clocks.
Determining Earth’s Age • Using information from these dating methods, geologists have divided Earth’s history into eras and periods. • Boundaries between the divisions are based on major differences in the fossil organisms contained in the layers. • The divisions were established before the actual ages of the eras and periods were known. • In the Precambrian era, early life evolved.
The Changing Face of Earth • Earth’s crust consists of solid plates that float on a fluid mantle. • The mantle is heated by energy from radioactive decay in the Earth’s core. Convection currents of mantle fluid cause the crust plates to move. • The process of plate movement is known as continental drift. • Throughout Earth’s history, the plates that carry the continents have drifted apart and moved back together numerous times. • Plate movement has affected climate, sea level, and the distribution of organisms.
The Changing Face of Earth • Earth’s atmosphere has also changed since the time the planet formed when little or no free oxygen was present. • Oxygen concentrations began to increase significantly about 2.5 billion years ago when some prokaryotes evolved the ability to split water as a source of hydrogen ions for photosynthesis. The waste product is O2. • One lineage of these oxygen-generating bacteria evolved into the cyanobacteria. These organisms formed rocklike structures called stromatolites. • The cyanobacteria liberated enough O2 to allow the evolution of oxidation reactions as the energy source for the synthesis of ATP.
The Changing Face of Earth • As life continued to evolve, the physical nature of the plant was irrevocably changed. • Living organisms not only added O2 to the atmosphere but also removed CO2 from it. • An atmosphere rich in O2 made possible the evolution of larger cells and more complex organisms. • About 1,500 mya, O2 concentrations became high enough for large eukaryotic cells to flourish and diversify. • By 750–700 mya, O2 had increased to levels that could support multicellular organisms.
The Changing Face of Earth • Unlike the unidirectional change in Earth’s atmospheric O2 content, most physical attributes on Earth have involved irregular oscillations. • External events such as collisions with meteorites have also affected Earth, sometimes resulting in mass extinctions.
The Changing Face of Earth • Climatic conditions have fluctuated through Earth’s history. • At times, Earth was colder than it is today; large areas were covered with glaciers at the end of the Precambrian and during the Carboniferous, Permian, and Quaternary periods. • Usually climates change slowly, but major climatic shifts have taken place over periods as short as 5,000 to 10,000 years. • For example, during one Quaternary interglacial period, the Antarctic Ocean changed from being ice-covered to being nearly ice-free in less than 100 years.
Figure 22.5 Hot/Humid and Cold/Dry Conditions Have Alternated Over Earth’s History
The Changing Face of Earth • Although most volcanic eruptions produce only local or short-lived effects, a few very large eruptions have had major consequences for life. • The collision of continents during the late Permian (about 275 mya) created a single, giant land mass called Pangea and caused massive volcanic eruptions. • Ash from the eruptions reduced the penetration of sunlight to Earth’s surface, lowering temperatures, reducing photosynthesis, and triggering massive glaciation.
The Changing Face of Earth • Collisions with large meteorites are rare, but they have been responsible for several mass extinctions. • Evidence for these collisions includes: • Impact craters • Rock disfigurations such as shocked quartz crystals • Helium and argon within giant molecules that have isotopic ratios characteristic of meteorites • Abundant fern fossils suggesting that meteorite impacts had scoured vast areas of Earth’s surface
The Changing Face of Earth • The first impact to be documented was that of a meteorite 10 km in diameter that caused a mass extinction at the end of the Cretaceous. • Abnormally high concentrations of iridium in a thin layer separating the Cretaceous and Tertiary rocks was found. • Iridium is very rare on Earth but abundant in some meteorites. • Then a 180-km-diameter crater buried beneath the northern coast of the Yucatán Peninsula of Mexico was discovered.
The Fossil Record • Fossils are a major source of information about changes on Earth during the remote past. • Periods of geological history are marked by mass extinctions or by dramatic increases in diversity called evolutionary radiations. • Evidence suggests that the major divisions in many animal lineages predate the end of the Precambrian by more than 100 million years. • Although the fossil record is fragmentary before 550 mya, it is still good enough to show that the total number of species and individuals increased dramatically in late Precambrian times.
The Fossil Record • An organism is most likely to become a fossil if its dead body is deposited in an environment that lacks oxygen. • About 300,000 species of fossil organisms have been described. • 1.7 million species of present-day biota have been named. • The actual number of living species is probably at least 10 million. • Most species exist, on average, for fewer than 10 million years; therefore, Earth’s species must have turned over many times during geological history.
The Fossil Record • Among the nine major animal groups with hard-shelled members, approximately 200,000 species have been described from fossils. • The fossil record is especially good for marine animals that had hard skeletons. • Insects and spiders are also well represented in the fossil record. • Combining data about physical events with evidence from the fossil record, scientists can compose pictures of what Earth and its inhabitants looked like at different times.
Major Patterns in the History of Life on Earth • For much of its history, life was confined to the oceans. • Shallow Precambrian seas teemed with life, including protists and algae. • By the late Precambrian, many kinds of soft-bodied invertebrates had evolved, some of which may be members of animal lineages that have no living descendants.
Major Patterns in the History of Life on Earth • By the early Cambrian period (543–490 mya), atmospheric O2 levels had nearly reached current levels. • The continental plates came together in several masses. Gondwana was the largest. • The rapid diversification of life that took place at this time is referred to as the Cambrian explosion. • The best fossils of Cambrian animals are found in China. • A mass extinction occurred at the end of the Cambrian period.
Major Patterns in the History of Life on Earth • During the Ordovician period (490–443 mya), the continents were mostly in the Southern Hemisphere. • Evolutionary radiation of marine organisms was intense. Animals lived on the sea floor or burrowed in sediments. • Ancestors of club mosses and horsetails colonized wet terrestrial environments. • At the end of the Ordovician, sea levels dropped about 50 meters, and glaciers formed over Gondwana. 75% of marine species became extinct.
Major Patterns in the History of Life on Earth • In the Silurian period (443-417 mya), northern continents coalesced, but their general position did not change. • Marine organisms rebounded from the Ordovician extinction, but few new species evolved. • The tropical sea was uninterrupted by land barriers; therefore, marine organisms dispersed widely. • The first known tracheophytes appeared on land in the late Silurian.
Major Patterns in the History of Life on Earth • During the Devonian period (417–354 mya), rates of evolutionary change accelerated. Land masses slowly moved northward. • Evolutionary radiation of marine animals such as coral and shelled cephalopods was high. • All current major groups of fishes were present by the end of the Devonian. • On land, club mosses, tree ferns, and horsetails became common and the first gymnosperms appeared. • Fishlike amphibians began to occupy the land. • At the end of the Devonian, 75% of marine species went extinct.
Major Patterns in the History of Life on Earth • The Carboniferous period (354–290 mya) was marked by large glaciers formed at high latitudes and extensive swamp forests grew on the tropical areas of the continents. • Fossilized remains of the forests formed the coal we now mine for energy. • Diversity of terrestrial animals increased greatly. • Snails, scorpions, centipedes, and insects were abundant. • Amphibians became larger, and reptiles evolved from one amphibian lineage. • Crinoids were plentiful on the seafloor.
Major Patterns in the History of Life on Earth • During the Permian (290–2458 mya), the continents coalesced into a supercontinent called Pangaea. • Massive volcanic eruptions poured lava over large areas of Earth. • Ash produced from the eruptions blocked sunlight and cooled the climate, resulting in the largest glaciers in Earth’s history. • By the end of the Permian, reptiles greatly outnumbered amphibians. • The lineage leading to mammals diverged from one line of reptiles. Bony fishes radiated in the oceans.
Major Patterns in the History of Life on Earth • At the end of the Permian, a large meteorite crashed into northwestern Australia. • Volcanic eruptions poured lava into the oceans, which depleted O2 in deep oceans. Oceanic turnover then carried the depleted water to the surface where it released toxic CO2 and H2S. • About 96% of all species on Earth became extinct.
Major Patterns in the History of Life on Earth • At the start of the Mesozoic era (248 mya), the few surviving organisms found themselves in a relatively empty world. • Pangaea slowly separated, glaciers melted, and shallow inland seas formed. • Life proliferated and diversified. • Earth’s biota diversified and became distinct on each continent.
Major Patterns in the History of Life on Earth • In the Triassic period (248–206 mya), vertebrate lineages became more diverse. • Conifers and seed ferns became the dominant trees. • Frogs and turtles appeared. • A great radiation of reptiles began, which gave rise to dinosaurs, crocodilians, and birds. • The end of the Triassic was marked by a mass extinction that eliminated 65% of the species on Earth. • A large meteor that crashed into Quebec may have been responsible.
Major Patterns in the History of Life on Earth • During the Jurassic (206–144 mya), two large continents formed—Laurasia in the north and Gondwana in the south. • Ray-finned fishes began the great radiation that culminated in their dominance of the oceans. • Salamanders and lizards first appeared. • Flying reptiles evolved. • Dinosaur lineages evolved into bipedal predators and quadrupedal herbivores. • Mammals first appeared.
Major Patterns in the History of Life on Earth • By the Cretaceous period (144–65 mya), Gondwana was beginning to break apart, and a continuous ocean circled the tropics. Sea levels were high and the Earth was warm and humid. • Flowering plants (angiosperms) evolved from gymnosperms. Many groups of mammals had evolved, but most were small. • Another mass extinction marked the end of the Cretaceous; it was probably caused by a large meteorite colliding with Earth. • All vertebrates larger than about 25 kg in body weight, including all of the dinosaurs, apparently became extinct as a result of this impact.
Figure 22.15 Positions of the Continents during the Cretaceous Period