Although fossils dated back only to the Cambrian during Darwin’s time, radiometric dating has since identified fossil bacteria as old as the beginning of the Archean era 3.5 billion years ago. The geologic record shows over 2 billion years during which the only life was unicellular. The appearance of eukarytoic cells roughly 1.8 billion years ago marked a dramatic increase in cellular complexity. In rocks 1 billion years old, multicellular eukaryotes begin to appear, and by the end of the Precambrian, fossils record a variety of ancient multicellular organisms. The beginning of Cambrian Period marks an “explosion” of life, and in general, biodiversity has increased throughout the Phanerozoic, shown in Figure below. Our current understanding of the fossil record confirms Darwin’s ideas that life began as tiny single-celled bacteria and over vast time evolved to produce the complexity and diversity we celebrate today.
As recorded in fossils, the evolution of life was not smooth or steady. Mass extinctions and episodic speciation interrupted the overall pattern of increasing biodiversity, shown in Figure below. These disruptions reflect dramatic changes in the environment of the Earth.
Figure 11.9
Estimates of numbers of marine genera throughout the last 542 million years support a gradual increase in biodiversity, interrupted by five major mass extinctions. Some scientists dispute the accuracy of such estimates, while others argue that they show regular cycles of extinction.
A major theme of the fossil record is loss of species. The death of a species – extinction – seems to be as much a characteristic of life as the death of individual organisms. Both seem closely linked to change in environment. Through mutation or sexual reproduction, offspring show variation. Individuals whose variations are not well suited to their environment die. Those whose variations are adaptive survive to reproduce. Death and differential reproduction result in adaptation to a changing environment.
These same forces of natural selection inevitably affect species: the fossil record indicates that up to 99.9% of all species that have ever lived on Earth are now extinct. Mass extinctions involve most major groups of organisms over a short period. The past 550 million years, when fossils are sufficiently abundant to tell a reliable story, show five mass extinctions in which more than 50% of animal species died. The most famous is the extinction which ended the reign of the dinosaurs 65 million years ago. Accelerated evolution may follow mass extinction, because the empty ecological niches make way for new species. After the non-birdlike dinosaurs disappeared, mammals rapidly evolved to fill the available niches. The fossil record shows numerous examples of episodic speciation, a pattern of periodic increase which includes these rebounds as well as bursts of evolution following major new “discoveries” or “ideas” – for example, the biochemical pathways for photosynthesis or cellular respiration.
Closely related to mass extinction is the theme of major environmental change throughout Earth’s history. Rock layers reflect critical changes in atmosphere and climate: oxidized iron deposits mark the introduction of oxygen gas to the atmosphere, and glacial deposits reflect periodic ice ages alternating with times of global warming. Craters and unique worldwide strata suggest that spectacular asteroid or comet collisions may have severely reduced solar radiation, and lava flows and ash suggests volcanism could have done the same. Massive geographic changes, now explained by plate tectonics theory, underlie volcanism as well as formation of new land bridges, seaways, and continents. Certain worldwide sedimentary deposits suggest significant sea level fluctuation, which may result from some of the aforementioned climate or plate tectonic changes. Life evolved against the backdrop of these often-catastrophic changes, and over 3.5 billion years of natural selection inevitably responded to them. Many of these changes are believed to have caused the mass extinctions and episodic speciation revealed in the fossil record. We will look at some of these events in more detail in the next two lessons.
Figure 11.10
Two theories of evolutionary change - gradualism vs. punctuated equilibrium - are still debated. The former proposes continuous change, while the latter suggests that species remain constant for long periods of time and that change, when it occurs, is rapid.
Two caveats are critical in interpreting the history of life using the Geologic Time Scale. The first concerns the idea that evolution progresses via slow, steady, gradual change. We have already seen that mass extinction and episodic speciation interrupt the overall pattern of increasing biodiversity, but gradualism suggests that changes accumulate continuously as one species evolves to become another. An alternative, more recent theory, punctuated equilibrium, shown in Figure above, proposes that species remain the same for long periods, and that change occurs infrequently but rather rapidly under unusual conditions such as geographic isolation or migration. The rather sudden appearance and disappearance of many individual species within the fossil record, noted even by Darwin, tends to support the latter theory. The idea of quantum evolution attempts to explain the origins of major groups (families, orders, and classes) as a response to drastic changes in environment or adaptive zones. The fossil record supports great variation in the rate of evolutionary change - from group to group and even among closely related lineages. Each of these ideas about pattern and rate may accurately describe one of many ways evolution works.
Figure 11.11
Charles Darwin's 1837 sketch from his First Notebook on Transmutation of Species (1837), shows the bush-like pattern of evolution.
A second caveat: the 4.6 billion year time scale makes it tempting to view evolution as linear, and perhaps even goal-directed. Time may be an arrow, but evolution is much more a bush of common ancestry–a family tree, as we saw at the beginning of the chapter. Darwin recognized this - his sketch, shown in Figure above, shows the pattern of speciation predicted by his theory of chance variation and adaptive selection. A very recent (August 2007) discovery encourages us to view our own human ancestry as a bush rather than a line. Radiometric dating of a new fossil of Homo habilis shows that this species coexisted with the "more advanced" Homo erectus, shown in Figure below. Previously, scientists considered the former an ancestor of the latter. The inappropriate expression “more advanced” implies the false, linear, goal-directed interpretation of evolution.
Figure 11.12
Homo habilis (left) was considered an ancestor to Homo erectus (right) until the 2007 discovery of a habilis fossil which showed that the two species coexisted. The history of the genus Homo, like the evolution of most species, is undoubtedly more bush-like than linear.
A famous example of “bushiness” in the history of life is adaptive radiation, a type of divergent evolution. This pattern of speciation involves the relatively rapid evolution from a single species to several species which fill a diversity of available ecological niches. Mass extinctions (the dinosaurs!), new volcanic islands (the Galapagos, or Hawaii), land bridge formation (the isthmus between North and South America) or “invention” of a new idea in evolution – all are events which “suddenly” open a variety of niches for adaptive radiation. In each case, a fundamental structure in one species is modified to serve new functions in different environments or modes of life. For example, forelimbs of mammals have become elongated with grasping hands for the forested habitats of monkeys, flattened into flippers for the aquatic habitats of whales, and spread into wings for the aerial habitats of bats, shown in Figure below. Adaptive radiation explains – and the fossil record shows – that these groups all arose from one ancestor or a small group of common ancestors.
Figure 11.13
Forelimbs of mammals show adaptive radiation, or divergent evolution. Evolution has modified the original pattern in a common ancestor to suit a multitude of different environments.
In contrast to divergent evolution, whereby closely related species evolve different traits, convergent evolution involves distantly related species evolving similar traits. This pattern surfaces frequently in the history of life when different organisms occupy similar eco
logical niches. For example, three major groups of organisms have evolved wings for flight: reptiles (pterosaurs), birds, and mammals (bats), shown in Figure below.
Figure 11.14
The wings of pterosaurs (1), bats (2) and birds (3) show convergent evolution. Similar structures adapt each group to flight, but each of the three types of wing evolved independently.
Figure 11.15
Australias fauna demonstrates the importance of geography to evolution. Mammals evolved in the northern hemisphere and migrated to Australia across a land bridge (see A, above) which was later submerged (B). Marsupials persisted and underwent adaptive radiation in Australia. Elsewhere, the appearance of placental mammals spelled doom for the marsupials; placental mammals outcompeted them and underwent their own adaptive radiations. These separate radiations (green, orange, and red in B) resulted in a number of examples of convergent evolution.
Australian fauna reveal both divergent and convergent patterns related to major geographical change, shown in Figure above. Major groups of mammals evolved in the northern hemisphere and migrated to Australia across a land bridge. Later submerging of the land bridge isolated the Australian mammals, and the marsupials underwent their own adaptive radiation within their insular continent. Elsewhere, placental mammals evolved to out-compete the more primitive monotremes and marsupials, and underwent their own adaptive radiation. These independent radiations resulted in some wonderful examples of convergent evolution: An example: the marsupial Tasmanian wolf (now extinct) shared with the placental canines many adaptations to life as a hunting predator, shown in Figure below.
Figure 11.16
The thylacine or Tasmanian wolf, a marsupial, closely resembles the golden jackal, a placental canine; both show similar adaptations to predatory life, demonstrating convergent evolution. Marsupials and placentals evolved independently due to the loss of a land bridge connecting Australia to southeast Asia, so they provide examples of convergent evolution.
One last, fascinating pattern within the history of life is coevolution. In coevolution, two species or groups of species influence each other’s evolution and therefore evolve in tandem. Relationships may be positive for one species or both, or an evolutionary arms race between predator and prey. Flowering plants depend on insects for pollination, so have evolved colors, shapes, scents, and even food supplies which are attractive to certain insect species. Insects, in turn, have evolved mouthparts, senses, and flight patterns which allow them to respond to and benefit from specific floral “offerings,” shown in Figure below. The Endosymbiotic Theory describes a special form of co-evolution: Mitochondria and chloroplasts evolve within eukaryote cells, yet because these organelles have their own DNA sequence, different from that of the nucleus in the “host” cell, organelle and host cell evolve in tandem – each influences the evolution of the other.
Figure 11.17
Impressive proboscis and vivid colors! Hawk moths and the zinneas influence each others evolution, because the flower depends on the moth for pollination, and the moth feeds on the flower.
Closely related to coevolution is coextinction. If one member of a pair of interdependent species becomes extinct, the other is likely to follow. Famous examples were two species of bird lice which were obligate parasites on the passenger pigeon, shown in Figure below. When “Martha,” a resident at the Cincinnati Zoo thought to be the last passenger pigeon in the world died on September 1, 1914, the extinction of the bird lice species followed. Alas, one louse species was later rediscovered on a band-tailed pigeon, and the other species had been misidentified.
Figure 11.18
The passenger pigeon and a parasitic species of louse (not the one pictured above) demonstrate coevolution and potential coextinction. Each species influenced the others evolution, and when the host became extinct in 1914, the parasite narrowly escaped extinction only because an alternate host the band-tailed pigeon survived.
In this lesson, you have explored the tools we use to study the history of life, the Geologic Time Scale which organizes what we know, and a variety of patterns found in the 4.6 billion year story. In the next lessons, look for examples of these patterns as we follow that story from the origin of life to what is becoming known as the Sixth Extinction today.
Lesson Summary
If the history of life is condensed into a single 24-hour “cosmological day,” humans occupy only the last minute and one-half, and civilization, less than the final second.
Fossils include mineralized remains of organisms, casts, impressions, footprints, burrows, droppings, eggs, or nests as well as frozen, dried, or amber-coated remains.
The positions of fossils in rocks indicate their relative ages; older fossils and rock layers are deeper than fossils and rocks that are more recent.
Radiometric dating measures the proportion of decay products of radioisotopes with known half-lives to estimate absolute age. Different isotopes with a range of half-lives cover the span of geologic time.
Comparing DNA or protein in similar species can reveal evolutionary relationships and confirm patterns suggested by the fossil record.
Geologic Time Scale divisions mark major events which highlight changes in climate, geography, atmosphere, and life.
The largest units of time are Eons; the 4.6 billion years of earth’s history are divided into four eons.
The Phanerozoic includes the most recent 545 million years and the most detailed fossil record.
Overall, the fossil record confirms Darwin’s idea that life began as tiny single-celled bacteria and over time evolved to produce the complexity and diversity we celebrate today.
Mass extinctions and episodic speciation interrupt an overall gradual increase in complexity and diversity.
Evolution shows response to major environmental changes, including volcanism, continental drift, sustained warming and cooling, asteroid impact, and critical transformations of earth’s atmosphere.
The rate of evolution is not always uniform; gradualism does not characterize all speciation.
The pattern of evolution is seldom linear, but rather more like a bush or family tree.
Punctuated equilibrium, quantum evolution, and variable rates describe patterns of evolution which may differ from gradualism.
In divergent evolution, also called adaptive radiation, closely related species evolve different traits to adapt to a variety of available niches.
In convergent evolution, distantly related species evolve similar traits as adaptations to similar habitats.
Geographic changes, including continental drift, affect patterns of evolution.
Review Questions
How do the conditions needed for fossilization explain the rarity of fossils?
Compare relative dating of fossils and rock layers to absolute dating.
Explain why “carbon dating” is an inadequate description of aging rocks and fossils.
Describe how molecular clocks clarify evolutionary relationships.
Compare and contrast Geologic Time with absolute time, including the limits of each.
Explain the ways in which the Geologic Time Scale and the fossils it records may be misleading concerning the history of life.
Construct a table or chart which shows five of the major patterns of macroevolution we have observed in the fossil record. Include the pattern name, a brief description or definition, causes or contributing factors (where applicable), and a specific example for each.
Explain how some of the patterns of evolution and environmental change account for worldwide differences in the distribution of mammals. Discuss placentals vs. marsupials.
Charles Darwin described an orchid from Madagascar that had a nectar well which measured 12 inches deep, keeping costly sugars far out of reach of all known butterflies and moths. He predicted the existence of a highly specialized pollinator moth with a foot-long proboscis that could act as a straw to reach the nectar. After Darwin’s death, scientists discovered a night-flying moth that matched Dar
win’s expectation and named it the “Predicta.” Which pattern of evolution did Darwin see in the orchid? Explain why this is a good example of the pattern.
Relate human history to life’s history, as shown in the Geologic Time Scale and fossil record.
Further Reading / Supplemental Links
Colleen Whitney, Kate Barton, David Smith, “The Paleontology Portal.” University of California Museum of Paleontology, Paleontological Society, Society of Vertebrate Paleontology, and US Geological Survey, 2003. Available on the web at:
http://www.paleoportal.org/index.php
“Continental Drift Animation.” EduMedia-sciences, 2002-2007. Available on the web at
http://www.edumedia-sciences.com/a95_l2-continental-drift.html
Dave Smith, “Life Has a History – Level 2.” University of California Museum of Paleontology, 7/18/06. Available on the web at:
http://www.ucmp.berkeley.edu/education/explorations/tours/intro/Intro5to12/tour1nav.php
Jim Kurpius. Rob Guralnick, Jennifer Johnson, Anne Monk,Judy Scotchmoor, and Mark Stefanski, “Understanding Geologic Time.” University of California Museum of Paleontology, 1994-2007. Available on the web at:
http://www.ucmp.berkeley.edu/education/explorations/tours/geotime/index.html
Lexi Krock, “The Missing Link: A Brief History of Life.” Nova Online, last updated February 2002. Available on the web at:
http://www.pbs.org/wgbh/nova/link/history.html
Roy Caldwell and David R. Lindberg, “Evolution 101.” University of California Museum of Paleontology, 2007. Available on the web at:
http://evolution.berkeley.edu/evolibrary/article/evo_01
CK-12 Biology I - Honors Page 46
