- Describe the processes necessary for fossilization and the importance of context in relationship to understanding the information fossils contain.
- Describe the many relative and chronometric dating techniques used in the sciences.
- List the temporal and anatomical characteristics of the different groups of fossil primates from the Cenozoic to Pleistocene.
- Analyze the climate of the Cenozoic as to its impact on animal and primate evolution, especially the origin and evolution of both monkeys and apes.
- Explain both molecular phylogeny and the molecular clock in relationship to our understanding of primate evolution.
Doom was coming out of the sky in the form of an enormous comet or asteroid—we are still not sure which it was. Probably 10 km across, traveling tens of kilometers a second, its energy of motion had the destructive capability of a hundred million hydrogen bombs. It is worth pondering the realization that each of us is descended from unknown ancestors who were alive on that day when the fatal rock fell from the sky. They survived and the dinosaurs did not, and that is the reason why we are here now—as individuals and as a species. That one terrible day undid the benefits which 150 million years of natural selection had conferred upon the dinosaurs, making them ever fitter to be the large land animals of Earth. Evolution had not equipped them to survive the environmental disasters inflicted by a huge impact and when the holocaust was over, they were gone.
Evolution had not provided impact resistance for the mammals either, but somehow they did survive. No one knows why, but it must have helped that they were smaller and therefore much more numerous than the dinosaurs, so that there was a better statistical chance that some would live.
When the environmental disruptions from the impact had waned and the mammal survivors emerged into a new world they must have faced great dangers and great opportunities. —W. Alvarez, T. rex and the Crater of Doom (1997, p.130)As the vignette suggests, geological forces have had a critical impact on mammalian evolution, including primate evolution. Paleontology, a field that takes its name from the Greek words for “old” (paleos) and “existence” (ontos), is devoted to finding, studying, and understanding fossils, the preserved remnants of once-living things. Paleontologists want to know how old the fossil is, what kind of organism it represents, how it lived, and how that fossil came to be preserved where it was.
we will set the stage for understanding human evolution by looking closely at the fields of geology, the study of the earth, and primate paleontology. We will see how materials fossilize and look at what we can learn from both the fossils themselves and the surroundings in which they are found. We’ll introduce and compare some of the most important dating methods in use today, and we’ll explore conditions on Earth during the Cenozoic Era, the time period in which primates evolved. We also consider what we know about the origin and evolution of the Primate order. We will look at three of the major events in primate evolution: the strepsirhine– haplorhine split, the origin of Old World and New World monkeys, and finally the origin of the apes.
To help organize the fossils, you will want to refer to the family tree of living primates and review the bony characteristics that allow us to recognize animals at different levels of that tree.
In each case, we focus on the anatomical characters of the fossils and the ecological circumstances in which they evolved, and we discuss possible scenarios for what this evidence tells us about the natural selective factors that favored the origin of each group.
How to Become a Fossil
You might think that fossils are abundant. After all, every organism eventually dies, and natural history museums are filled with fossils of dinosaurs and other prehistoric creatures. In reality, very few living things become fossils and only an exceedingly small proportion of these fossils are discovered, collected, and studied. Thus, the fossil record is not entirely representative of the composition of past biological communities (Behrensmeyer and Hill, 1980). Instead, the fossil record preserves some organisms in abundance, whereas others are seldom preserved.
Taphonomy, the study of what happens to remains from death to discovery, reveals some of the factors that determine whether an organism becomes a fossil (Shipman, 1981). These include both biological and geological processes. Death might come to a human ancestor or any other animal in a number of ways, such as old age, injury, disease, or predation (Figure 9.1). In many instances, the agent of death may leave marks on the skeleton, such as the bite marks of a predator. After death, the carcass begins to decompose and numerous microbes, bacteria, mold, and insects, accelerate this process. While this is happening, scavengers may ravage the carcass, consuming its soft tissues and perhaps even chomping on its bones.
Eventually, only the most durable tissues remain, especially the densely constructed middle shafts of the limb bones, the jaws, and the teeth (Brain, 1981). Even these durable remains can disappear through various means including erosion and trampling.
To become a fossil, part of the organism must be preserved by burial, a natural process in which the carcass or part of it is covered with sediment. Burial interrupts the biological phase of decomposition, protecting the skeleton from further trampling.Burial often occurs in the floodplains of rivers, along the shores of lakes, and in swamps where uplift, erosion, and sedimentation are occurring. Once buried, skeletal remains may absorb minerals from the surrounding soil or ground water, which eventually
replace the organism’s original inorganic tissues. The result is petrifaction the process of being turned to stone. On occasion soft parts such as skin, hair, or plant parts may be preserved. In very exceptional circumstances, the original tissues of an organism are preserved largely intact. For example, whole mammoths have been found frozen in permafrost and naturally mummified people have been found, especially in arid places.
Finally, trace fossils such as the tracks left by animals may provide impressions of their activities, and coprolites, or fossilized feces, also tell us about the presence of past animals.
The Earth in the Cenozoic
Having established the various ways we might assess the age of a paleontological site and the fossils within it, we now turn to other issues of understanding the context in which fossil primates are found. Most importantly, we will look at the position of the major land masses during the Cenozoic, which has implications for how animals moved from one place to another, and then we consider the various methods scientists use to reconstruct the habitat in which animals once lived.
Continents and Land Masses
As you may be aware, the continents have not always been in their current locations. Approximately 200 million years ago the earth was divided into two major land masses that we now call Laurasia and Gondwanaland. Laurasia was composed of most of present-day North America and Asia, and Gondwanaland included Africa and South America . By 50 million years ago North America and Asia were beginning to spread apart, and both South America and Africa had separated from one another and from the other continents. Africa eventually became connected to Asia via the Near East, North America and Asia were separated by a chain of islands (but remained connected during low sea levels), and South America was an island continent until well into the Pliocene (~3.5 million years ago), when the Central American land bridge connected it to North America. These movements are critical for understanding early primate evolution, particularly the distribution of the Eocene primates and the conundrum of the origin of the South American primates (which appeared while that continent was still an island). Once the continents were in their present positions, the onset of severe glacial events in the late Piocene and Pleistocene periodically lowered sea levels, exposing additional land and sometimes resulting, as is the case between continental Asia and Indonesia, in land bridges between otherwise isolated areas .
The Environment in the Cenozoic
Conditions in the environment naturally select individuals most suited to them, and because of their favored traits, these individuals reproduce more than others in the population. So, studying past environmental conditions is critical to understanding the selective pressures affecting the survival and extinction of animals in the past. We can reconstruct environmental conditions from several kinds of geological and biological evidence. Here we consider ways for reconstructing past temperature, sea levels, and animal and plant communities.
Oxygen Isotopes, Temperature, and Sea Level Perhaps the best-known climate proxies are oxygen isotope curves that rely on the ratio of stable oxygen isotopes in the past as a proxy for global temperature and sea level. The process works like this. Two stable isotopes of oxygen, 16O and 18O, differ in weight, with 18O being the heavier of the two. These isotopes exist as oxygen in water molecules and other compounds. In water they are incorporated into the shells of marine invertebrates that are composed of calcium carbonates. Water molecules formed of the lighter isotope tend to float nearer the ocean surface, and water molecules formed of the heavier isotope tend to sink; therefore, the lighter isotope of oxygen tends to evaporate from ocean surfaces sooner than does 18O. During cold
periods when 16O evaporates from the ocean, it is not returned to the world’s water reserves via rain but is locked up in ice at the poles and northern latitudes. Consequently, sea levels are lower during cold periods and contain a greater percentage of 18O than during warmer periods. Therefore, the 18O/16O ratio increases in sea water during cold periods and in the shells of the marine animals formed in them at that time .
The Pliocene and Pleistocene epochs are characterized by oscillations in temperature from colder (glacial) to milder (interglacial) periods. Oxygen isotope curves have been important for reconstructing climate patterns in the mid- and late Pleistocene and correlating the movements of Neandertals and modern humans in relation to climate change. Global climate patterns can help us understand what kinds of conditions animals lived in during the past. But local differences in climate would also exist within these global patterns—for example, think of the differences in climate between the beach and the mountains today. Plant and animal fossils from specific paleontological localities help us understand these more local environmental conditions.
Vegetation
Fossilized plants, pollens, and plant impressions can show us what the local environment was like at the time a paleontological site was formed. Plants are less often preserved than bones, but under certain circumstances, such as in peat bogs or very fine-grained sediments, plant fossils are abundant. These fossils can be used to compare the environments that animals once lived in with those of today. For example, the recovery of fossil pollens can tell us about the presence of certain kinds of plants in an area. The Neandertal site of Shanidar was thought to show the ritual burial of a Neandertal on a blanket of flowers (based on the plentiful pollen around the skeleton). But it is also possible that modern pollens were introduced to the site, either as the archaeologists excavated or as the wind blew over local plants.
Stable Carbon Isotope Ratios in Teeth and Soil Stable
carbon isotope ratios are also used to reconstruct the types of vegetation in a region. We can use carbon isotopes to differentiate between plants using different photosynthetic pathways because these different pathways lead to the retention of different amounts of carbon isotopes. Trees and shrubs, different kinds of grasses, and arid-adapted plants use different photosynthetic pathways. By examining ratios of stable carbon isotopes in ancient soils (paleosols), scientists can identify if an ancient environment was, for example, an open grassland or a shady woodland. Analyzing paleosols in this way has been important in reconstructing environments in Africa during hominin evolution . We used to think bipedality arose in an open savanna environment, perhaps implicating heat stress or other selective factors in its origin. However, we are learning that many of the early hominin environments were more wooded, suggesting that another selective factor was at work; perhaps bipedality was an efficient means of crossing short distances between food patches while also carrying food. The recent publications on Ardipithecus ramidus have used soil carbonates as one piece of evidence to argue for a moister and more wooded environment for this early putative hominin (White et al., 2010); however, other analysts argue that these data indicate less than 25% tree cover (Cerling et al., 2010). Animal Communities Animal bones can also be used to infer local environment.
Although some types of animals seem to be able to live just about anywhere, most have preferred types of habitats. Hippos and crocodiles live near water sources, and the presence of monkeys usually indicates wooded areas. Animals that are adapted to running long distances over open terrain tend to have longer, slighter limbs; those adapted to life in forested areas often have shorter limbs. Based on comparisons with the adaptations in living animals of known habitat preference, paleontologists infer the climatic and environmental preferences of past animals associated with fossil primate and hominin sites and thus the paleoenvironmental conditions in which these primates lived.
Climate Change and Early Primate Evolution
Using the kinds of reconstructions described here, scientists have drawn a general picture of the climate during the evolution of the Primate order.
provides an overview of temperature changes throughout the Cenozoic. The story of Cenozoic climate change is generally one of cooling and drying. By combining these reconstructions of ancient climate change with information from the Primate fossil record, we can begin to understand how our lineage evolved.
The origin of primates is tied first to the origin of mammals, which began in the Mesozoic Era (225–65 million years ago), an age dominated by dinosaurs. At the end of the Mesozoic, drastic environmental changes, probably arising from an asteroid or comet crashing into the surface of the earth, caused or contributed to the extinction of the dinosaurs and generated opportunities for mammals (Alvarez et al., 1980). Evidence of such an impact comes from a giant crater called Chicxulub in the Yucatán Peninsula. The impact probably caused an all-consuming firestorm and a number of tidal waves, followed by abrupt global cooling. It is thought that this combination of fire and cold killed off much of the terrestrial plant life at that time, which caused the extinction of herbivorous dinosaurs and then also of the carnivorous dinosaurs that fed on them.
The ensuing environmental and ecological circumstances, including the absence of large prey animals, favored small, insect-eating mammals over the larger dinosaurs. Some of the primitive mammals of the Mesozoic persisted into the Paleocene, the earliest Cenozoic epoch, but for the most part there is a comprehensive replacement of mammals at the K–T boundary. Many of these new mammals are archaic forms that are not traceable to living groups. Such is the case with the possible ancestors of the primates.
Changes in the Paleocene: The Origin of Primates?
The Cenozoic began much, much warmer than it is today. The Paleocene and early Eocene were by far the warmest epochs of the Cenozoic, and temperatures differed less between the equator and the North and South poles than they do today. Thus, when primates first arose, not only were they equatorial and subequatorial animals, as they largely are today, but they existed fairly far north and south as well. Although it was warmer than today, there was some climatic fluctuation during each epoch. As the era proceeded, the climate cooled and dried but still fluctuated somewhat.