1) Introduction (post a constructive thought, question, or comment to this thread)
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’ (pathos) and ‘existence” (clues), is devoted to finding, studying, and understanding fame; 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. In this chapter, 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—haplorhinc 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 in Chapter 7 (Figure 7.9 on page 147) and review the bony characteristics that allow use 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.
2) How to become a Fossil(post a constructive thought, question, or comment to this thread)
You might think that fossils arc 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 (Behrensmcycr and Hill, 1980). Instead, the fossil record presents 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.
3)The Importance of Context(post a constructive thought, question, or comment to this thread)
A fossil without its context is useless, except perhaps as a pretty object on the mantelpiece. In this section we review the important principles used in geology to under-stand the position of a fossil in its rock layers and the relationship of different fossil sites to each other.
Imagine driving through a road cut where you see what looks like layers or hands of rock. These arc strata, literally layers” in Latin. In some road cuts these layers are basically horizontal, but in others they may be more vertical or even quite deformed (Figure 9.2). Stratigraphy is the study of the distribution of these layers. In 1830, Charles Lyon, whose work influenced Darwin (see Chapter 1), synthesized a number of accepted geological principles including the principles of stratigraphy. Four of these principles are critical to an understanding of the context of a fossil: original horizontality, superposition, cross-cutting relationships, and faunal succession. The rinei3Ole of original horizon:44y formulated by Nicolas Steno, says that layers of rock (strata) are laid down parallel to the earth’s gravitational field and thus horizontal to the earth’s surface, at least originally (Figure 9.3 on page 200). All the deformations and upendings that you see in road cups are caused by later activity such as earthquakes and volcanic eruptions (Figure 9.2h).
4)How Olds It?(post a constructive thought, question, or comment to this thread)
How do we know where in the geologic time scale a site and the fossils within it fall? A vital tint step in determining the antiquity of fossil remains is learning their provenience, the precise location from which the fossils come. After we have established provenience, we an apply a wide variety of techniques to estimating their age. There are three main ways to estimate age.
Relative Dating Techniques
Relative dating techniques use the principles of stratigraphy to tell us how old something is in relation to something else without applying an actual chronological age. If you sayyou have an older brother, we knowyour relative ages even though we do not know whether the two of you are 6 and 16 years old, 19 and 25, or 60 and 65. These techniques include litho,ttratigraphys tephrostratigraphy, hiostratigraphy, and chemical methods.
Lithostratigraphy uses the characteristics of the rock layers themselves to correlate across regions (litho refers to rock). For example, if millions of years ago a layer of limestone was formed by an inland sea that extended over a large area of West Virginia, across Pennsylvania, and into New York, then we would expect to see the limestone layer in all these areas even if there are different sequences of rock layers above and below the limestone. Therefore, the limestone layer allows to to correlate the widely separated sequences of rock layers (Figure 9.7 on page 206).
5)The Earth in the Cenozoic(post a constructive thought, question, or comment to this thread)
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 (Figure 9.14). 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 year 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 Pioccnc 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 .
6)Climate Change & Early Primate Evolution(post a constructive thought, question, or comment to this thread)
Using the kinds of reconstructions described here, scientists have drawn a general picture of the climate during the evolution of the Primate order. Figure 9.17 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 frail 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 Chi club in the Yucatan 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 dino-saurs. 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 replace-ment 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.