Dating back to the early history of science, it was long thought that Earth was a static, stable planet whose surface remained largely unchanged through time. This view radically changed during the 1960s, as an array of improved analytical techniques and an influx of new observations revealed that Earth’s surface is in a state of constant change. This new approach to understanding the Earth is known as plate tectonics and is composed of two basic processes: sea floor spreading and continental drift. Although these two processes are coupled, the notion of continental drift has allowed scientists to understand the evolution and distribution of many plant and animal groups, including primates.
The first suggestions of continental drift were offered by 16th-century philosophers and geographers, who noted the congruence between the coastlines of Africa and South America. In 1596, the geographer Abraham Ortelius argued that the Americas were once conjoined with Europe and Asia, but later “torn away” by earthquakes and other catastrophes. In recent years, historians of science have revealed nascent hints of continental drift in the writings of Francis Bacon and noted French scientist Comte de Buffon. However, it wasn’t until the early 20th century that a coherent hypothesis of continental drift was presented to the scientific community. This hypothesis was articulated by Alfred Lothar Wegener, a German meteorologist who assembled widely divergent lines of evidence into an understandable theory of continental motion.
Like the early geographers before him, Wegener was intrigued by the closely matching coasts of South America and Africa. After reading a paper describing similar Paleozoic fossils from these two continents, Wegener launched a massive literature search in the hopes of finding additional data to support continental drift. The data he uncovered were varied and wide-ranging. Wegener discovered that South America, Africa, India, Australia, and Antarctica shared a suite of unique Mesozoic fossils, including a signature fern flora and several reptiles. Modern animals do not range across all continents, because it is often impossible to disperse across oceans and other barriers. This suggested to Wegener that these continents were linked during the Mesozoic and have since moved to their present, widely divergent positions. Other evidence gathered by Wegener included closely matching rock units shared by Africa and South America and geological evidence indicative of former equatorial climate belts and past glaciations that made little sense if the continents have always occupied the same positions.
Wegener presented his hypothesis in a series of lectures and journal articles in 1912. Three years later, he outlined his ideas in a short, 94-page book, Die Entstehung der Kontinente und Ozeane, which was subsequently revised three times and translated into English as The Origin of Continents and Oceans. The notion of continental drift, which overturned much of the conventional geological wisdom of the day, was initially dismissed by critics as untenable, largely because Wegener could provide no plausible mechanism for continental motion. When Wegener died during a 1930 expedition to Greenland, his hypothesis was openly ridiculed and his scientific credibility scorned.
Although Wegener would never know it, his hypothesis was later verified as a new age of science dawned in the shadow of World War II. Over the course of the 1960s, a handful of earth scientists from across the globe instituted a scientific “revolution” that saw the birth of plate tectonics. Important data supporting this new theory came from studies of paleomagnetism. As lava cools into solid rock, tiny crystals of the magnetic mineral magnetite are “locked” into position, thereby recording the strength and direction of Earth’s magnetic field at the time of the rock’s formation. Using trigonometric equations, geologists can take this data and determine the latitude at which a certain rock formed. This procedure was applied to igneous rocks across the globe, and it was discovered that the latitudes at which the rocks formed were different than the latitudes they occupy today. This, along with an avalanche of additional data, strongly supported Wegener’s original hypothesis of continental drift, which today is regarded as an important tenant of plate tectonics.
Although important to geologists and other Earth scientists, continental drift is also relevant to studies of primate evolution and distribution. Understanding the patterns of continental motion greatly enhances our comprehension of biogeography, the study of the distribution of living species. As continents move over time, they carry with them living organisms, which evolve as the continents collide and drift apart. The distribution of both fossil and living primates, including members of the human lineage, is illuminated by continental drift.
Primates originated between 85 and 65 million years ago, at the same time that the southern landmass of Gondwana, comprising present-day Africa, South
America, Australia, Antarctica, India, and Madagascar, was rapidly fragmenting. Thus, it is likely that the initial evolution and divergence of primates was closely linked with continental drift. The extinct adapoids, a group of primitive primates, are only known from North America, Europe, Asia, and Africa, which is explained by a long-lived land connection between North America and Europe. Continental drift also helps explains the split between Old World and New World primates, including how early human ancestors evolved on an isolated African-Arabian continent and later dispersed after collision with Asia. The distribution of the more primitive prosimians is also elucidated by continental drift, but is somewhat complicated. Patterns of continental motion help explain why prosimians are limited to Africa, Asia, and Madagascar, but do not adequately describe why Madagascar is home to an abundance of species. Several hypotheses have attempted to answer this puzzle, many relying on continental drift, but there is no current consensus among researchers. Interestingly, purely geological processes associated with continental drift have enabled the preservation of important hominid fossils in Africa. The African continent is currently rifting, or splitting in two, as is manifested by the Red Sea and the line of East African lakes, such as Victoria and Malawi. This rifting is associated with volcanism and the formation of valleys, which have enabled preservation of early human fossils in Kenya and surrounding countries.
Continental drift has both revolutionized geology and enabled a better understanding of plant and animal distribution. Primates originated and evolved as continents moved and collided, which has affected modern primate distribution and habitats. As older primate fossils are discovered and more is discerned about the evolution of primate groups, the patterns of continental drift are becoming increasingly important to primatologists.
References:
- Hallam, A. (1973). A revolution in the earth sciences. Oxford: Oxford University Press.
- Hartwig, W. C. (Ed.). (2002). The primate fossil record. Cambridge: Cambridge University Press.
- Wegener, A. L. (1924). The origin of continents and oceans. New York: Dutton.
