ANT2511- The Human Species

Transcription

1 ANT2511- The Human Species Class Notes Introduction to Biological Anthropology (08-22) What is Anthropology? Topic placeholder o Material culture and tools o Speech and language o Hunting and food production o We are capable of abstract thought and other primates are not Culture o Learned behavior that Is transmitted from person to person o Culture is learned, it has no genetic or biological basis o A human universal: includes strategies for dealing with the natural environment. We cannot live without it. o Cultural relativity: each culture should be understood within its own context and not judged by other cultures norms Cannibalism (New Guinea, 20 th century) Gender roles (Saudi Arabia, 21 st century) o Is pervasive o All humans are ethnocentric Module 1 Webcourses Notes: Evolution: Constructing a Fundamental Scientific Theory 1 Questions to be addressed in this chapter: o How did the theory of evolution come to be? o What was Darwin s contribution to the theory of evolution? o What has happened since Darwin in the development of our understanding of evolution? o All living organisms on Earth are related through common ancestry. But, how did this all happen? And how do we know this? In this lecture, we will explore the history of scientific thinking that has led to our current understanding of biological evolution. Charles Darwin, shown in this slide, obviously features quite prominently in this lecture. However, Darwin did not come up with biological evolution out of the blue. His ideas were strongly influenced by many thinkers from many different scientific disciplines. In this lecture, we will look into the following questions: 1) How did the theory of evolution come to be? 2) What was Darwin s contribution to the theory of evolution? In other words, why is he so famous? And 3)

2 2 What has happened since Darwin in the development of our understanding of evolution? Let s start by going back to 1831 and joining Charles Darwin on the HMS Beagle. Charles Darwin and the HMS Beagle o As we discussed in the previous chapter, scientists start by making observations and asking questions. The story of Charles Darwin and the theory of evolution is no different. In 1831, a young (22-year-old) Charles Darwin joined the crew of the HMS Beagle for a 5-year voyage around the world. The Beagle sailed from England, around the tip of South America, to the Galápagos Islands, around the southern coasts of Australia and Africa, and back to England. During this journey, Darwin collected thousands of samples: plants, animals, fossils, and rocks. He made careful and important observations about the organisms he encountered. Descent with Modification o Why did different species live on different islands? How did this happen? Natural selection Common ancestry o Adaptive radiation o For instance, Darwin spent a considerable amount of time studying the plant and animal life the inhabited the Galápagos islands. In particular, Darwin noticed that different kinds of birds finches, in particular were quite variable, and different kinds of finches lived on different islands. But, their variation was not random: It appeared to vary according to the habitat in which they lived. In other words, particular birds were adapted for very specific environments. He was puzzled by this pattern of variation...and intrigued. o Great scientists ask great questions and Darwin was a great scientist. He wondered about many things he saw during his 5-year journey on the HMS Beagle. Those birds, for example, puzzled Darwin. Here is an image of four different species of finch that Darwin collected on different islands in the Galápagos archipelago. Darwin observed that these different bird species were adapted to eat very particular foods and live in very particular environments on those islands. How did this happen? As we ll see, Darwin surmised that birds with certain physical features, those that allowed them to eat certain foods in their environment, survived and reproduced. Through reproduction, they could pass on these physical features to their offspring, thus increasing the frequency of these features in the overall population of birds. In different environments, different physical features would be favored and would be selected. This idea, termed natural selection, is the driver of evolution, or, as Darwin phrased it, descent with modification. From a single common ancestor, many different kinds, or species, of birds can evolve because different physical features are favored in different environments. The 13 different finch species that currently live on the Galápagos represent a kind of rapid and prolific speciation known as an adaptive radiation. Before Darwin o Common views: Earth young Species divinely created Species immutable

3 3 o Emerging scientific views: Earth old Earth s surface has changes over time Plants and animals have changes o Some ologies for us Geology: study of earth James Hutton ( ) o Scottish geologist o Geological strata o Uniformitarianism Charles Lyell ( ) o Evidence for the antiquity of Earth Our species has figured out that the planet Earth is 4.6 billion years old. This is a remarkable achievement. But, of course, we have not always known this. In fact, in the 1600s, any suggestions that the Earth was more than just a few thousand years old were not only considered incorrect but blasphemous. However, this way of thinking began to change in the eighteenth century thanks, in part, to the work of the Scottish geologist James Hutton, shown in the upper left image. Hutton recognized that wind and rain caused erosion and formed sand, small rocks, and soil. These particles could then be redeposited, forming the layered pattern of rock we call strata (as shown so beautifully in the bottom image of Utah s Bryce Canyon). However, a few thousand years was simply not enough time for this process to occur: It would require millions of years. Hutton s ideas of geological strata and time depth rely on the assumption that the processes that occur today are the same ones that have occurred in the past. This is known as uniformitarianism, an idea that is widely accepted in all scientific fields today. Hutton s ideas were soon tested by the great geologist Charles Lyell (shown in the upper right), who confirmed that it would take millions, not thousands, of years for the Earth s geological strata to form. Paleontology: study of fossils Robert Hooke ( ) o Fossil wood was once alive Georges Cuvier ( ) o Organisms can go extinct o Different fossils in different geological strata Not only is the Earth old, but there were once organisms that populated the Earth that no longer exist. In fact, most of the life that has ever existed on our planet has gone extinct. So, how do we know it existed at all? Fossils. Robert Hooke, shown in the top image, invented the microscope, and examined fossil wood under his new device. He noted that the cellular structure of the fossil wood was the same as wood that exists today. In other words, he discovered that fossils were the remains of things that were once alive. Over 100 years later, the French naturalist Georges Cuvier, shown below, made two other important

4 4 observations. First, he proposed that fossils were from organisms that no longer existed. For instance, the fossil jaws shown here were not from elephants but from extinct elephant-like creatures called mammoths. This idea, which sounds quite obvious today, was not at all at the time. Many thought that extinction was impossible, since each organism was divinely created. Cuvier also discovered that different fossils could be found in different geological strata. If geological strata were layered by age, with the older sediments deeper and the younger ones more superficial, then the different fossils in the different layers of rock could demonstrate evolutionary change. Of course, we use this rationale today, but Cuvier did not. Instead, he proposed that the different layers represented groups of organisms that had been wiped out in a series of catastrophic events. Taxonomy and Systematics: study and classification and relationships of organisms Carolus Linnaeus ( ) o Naming system for all living organisms o Clustered hierarchy o Kingdom, Phylum, Class, Order, Family, Genus, Species Let s resume with an examination of the lineages that did survive such an event. Prior to Cuvier s work on fossils, the great Swedish naturalist Carolus Linnaeus (right) devised a system for naming and classifying all living organisms. His system is still used today and allows scientists from all over the world, speaking different languages, to understand one another. Linnaeus proposed that each species should receive a unique name composed of a genus and a species. For instance, humans are Homo sapiens. Chimpanzees are Pan troglodytes. Some of these names are familiar to you. The snake boa constrictor has the Linnaeus classification Boa constrictor, and the gorilla is Gorilla gorilla. Linnaeus also devised a hierarchical classification scheme in which all living organisms could be placed within a kingdom, phylum, class, order, family, genus, and, finally, species. The Linnaeus system of classification reveals that living organisms, including humans, are clustered in distinct ways, ways that could only be explained if these living organisms shared a common ancestor. Let s look at this in more detail using the human as an example. Here is the classification scheme for humans (Homo sapiens). Additional categories have been added, since not all living organisms fit neatly into the seven categories proposed by Linnaeus. Let s work from the bottom up. Humans, as already discussed, are Homo sapiens. We are the only living members of our genus Homo, though other species in this genus are known from fossils. We are members of the family Hominidae and the superfamily Hominoidea. However, we are not alone in this superfamily. Chimpanzees, gorillas, orangutans, and gibbons are also in the superfamily Hominoidea. They are in this superfamily because they share features with us, such as large brains, mobile shoulders, and an upright posture. What is important to recognize here is that this is not a coincidence. These other animals reside in our superfamily because we share a common ancestor with them. However, Linnaeus didn t know this. We are in the infraorder Anthropoidea, along with the monkeys, and the order Primates. Even Linnaeus recognized that humans were primates, not because he had access to DNA (he did not), but because we share a number of physical features. We are in the class Mammalia because we have body hair and nurse our young with milk. Again,

5 5 this is not a coincidence. We are mammals and share features with other mammals because we share a common ancestor with other mammals. Our vertebral column puts us in the phylum Chordata with fish, frogs, and all other vertebrates. We reside in the kingdom Animalia with all other multicellular organisms that consume other organisms for food. At this level, we are related to sponges and fruit flies. Make sure you recognize the importance of this pattern of nested hierarchies. This pattern of life on Earth is precisely what one would expect if life has evolved from common ancestry through time. In other words, Linnaeus work led Darwin to recognize that if all life was separately and divinely created, it was done so in a manner that completely mimics descent with modification, or evolution. Demography: study of populations But, how do complex organisms change over time? How do they evolve? Darwin s great insight of natural selection was strongly influenced, oddly, by an economist. Thomas Malthus, shown here, wrote a book entitled An Essay on the Principle of Population that laid the foundation for many of Darwin s ideas. Malthus observed that humans often have more than two offspring. If parents (two people) continued to have more than two children, then the population of humans would grow. In fact, Malthus argued that the growth could be exponential, resulting in billions and billions of humans in a short period of time. Although Malthus observed population growth and the crowded urban centers of the 1800s, like London, human population did not grow without limit. Why not? Malthus observed that the population of humans grows more slowly than expected because there is not enough food for everyone. Populations are limited by their resources. Therefore, there is a struggle for existence, with only certain individuals surviving and reproducing. Notice how this observation by Malthus, combined with Darwin s recognition that there is considerable variation in a population, forms the basis for natural selection. Evolutionary biology: study of organisms and their changes over time o Great ideas in science rarely emerge without considerable contributions from previous generations of great thinkers, and natural selection is no exception. To understand how natural selection was conceived, it helps to go back in time and consider what people thought about the natural world prior to Charles Darwin. The Judeo-Christian view of the world held that the Earth was quite young (~6,000 years old). Species had all been created by God and these species were unchangeable (immutable). However, scientists in the eighteenth and nineteenth centuries began to change this way of thinking. They began to collect evidence that, in fact, the Earth was quite old (millions of years old), and that the Earth s surface and its plants and animals have changed considerably over time. Evidence for these revolutionary ideas came from many different scientific arenas. We will look at how Darwin was influenced by new discoveries in the following sciences: geology, the study of the Earth and its processes; paleontology, the study of fossils; taxonomy and systematics, the study of organisms, their classification, and relationships to one another; demography, the study of populations; and evolutionary biology, the study of how organisms change over time. o We will next explore the discoveries of scientists who lived hundreds of years ago. Why are we doing this? Well, there is little doubt that someone eventually would have come up with the idea of natural selection. But, if it were not for the scientists we are about to meet, there is little chance that natural selection would have been discovered by Charles Darwin.

6 6 Lamarckianism o Darwin did not invent evolution Jean-Baptiste Lamarck ( ) Inheritance of acquired characteristics o It is a common misunderstanding that Charles Darwin invented evolution. But, the idea that living organisms have changed over time was already around in Darwin s time. Most notably, the French scientist Jean-Baptiste Lamarck argued that plants and animals had changed over time, or evolved. Even Darwin s grandfather, Erasmus, thought that species have changed over time. If scientists before Darwin already discovered that species can evolve, then why is Darwin so famous? What Darwin contributed to science was not just the evidence that life on Earth had evolved, but he proposed a mechanism for how this can happen: natural selection. Lamarck also proposed a mechanism: inheritance of acquired characteristics. But, this mechanism has been shown to be incorrect. Here s how Lamarck s version of evolution works: Giraffes once had a short-necked ancestor. To reach higher and higher branches, giraffes stretch their necks. Adult giraffes who have stretched out their necks will pass on this acquired feature of a long neck to their offspring. Over time, necks get longer and longer in the giraffe lineage. Now, compare this to Darwin s mechanism: natural selection. There is variation in an ancient giraffe population with some individuals having longer necks than others. They all feed on high branches. Those with longer necks eat, survive, reproduce, and pass on the trait of a long neck to the next generation. Those with shorter necks cannot reach as much food, are weak, and may even starve and die. The frequency of longnecked individuals in the population increases over time. What did Darwin actually do? o Lyell and Hutton: Earth old Gradualism o Cuvier: Fossils resemble living forms o Malthus: Ideas about reproduction, populations, and variation o Linnaeus and Lamarck: Species related Species can change o After his voyage on the HMS Beagle, Darwin spent the next several decades on an intellectual voyage. Unlike his first trip, this one took place almost entirely in one place: his study in his home, Down House, in Kent, England (shown on the right). There, Darwin did many experiments, collected vast amounts of evidence, and eventually organized his thoughts into a book entitled On the Origin of Species. In this book, Darwin proposed natural selection as the mechanism for evolution. As we have seen, from the work done by the great geologists Hutton and Lyell, Darwin knew that the Earth was quite old and that natural processes often occur gradually. Darwin read the work of Cuvier, and recognized that many of the fossils that he collected while sailing around the world resembled living forms. From Malthus, Darwin began to recognize the importance of reproduction, variation, and population-level thinking. And from Linnaeus and Lamarck,

7 7 Darwin realized that life was clustered into related, nested hierarchies, and that it had changed over time. These ideas led to his two great realizations: 1) All life on Earth is related through common ancestry. More related organisms share a more recent common ancestor; and 2) the mechanism for evolutionary change is natural selection. All he had to do now was publish these ideas. Natural Selection o Alfred Russel Wallace ( ) Independently discovered natural selection o On the Origin of Species (1859) o In 1858, Charles Darwin received a startling letter from the naturalist Alfred Russel Wallace (shown on right). Like Darwin, Wallace made wonderful observations about the natural world and wondered why living organisms looked as they did. Like Darwin, Wallace made many of his observations on islands in Wallace s case, the Indonesian archipelago. Wallace collected many samples, read the works of Lyell and Cuvier, and thought deeply about the mechanism of evolutionary change. In 1858, Wallace wrote to Darwin and proposed a mechanism for evolution that was strikingly similar to Darwin s own ideas. They agreed to jointly present their findings to the Linnaean Society of London in July However, it is Darwin who went on to publish On the Origin of Species, which presented the details of natural selection and the evidence for evolution, the following year. The fact that two scientists, working independently and on opposite sides of the world, both came up with the same mechanism for evolution, is robust evidence that natural selection is a powerful tool for explaining the pattern of life. Mechanism of Inheritance o Gregor Mendel ( ) Laws of inheritance No blending inheritance Discrete units: genes Different versions: alleles Dominant and recessive o Natural selection can only operate if there is variation. If every member of a species is the same size, or the same color, or has the same-sized beaks, then how can certain individuals survive and not others? Variation is a fundamental tenet to natural selection. But, this raises a new question: Where does variation come from? Why do individuals in a species look a bit different from one another? And why do closely related individuals look more similar to one another? Darwin and other scientists recognized that offspring looked like their parents. However, it was not known how this actually happens. Clearly something gets from the parents to the offspring, but what? And how does this work? During Darwin s time, it was though that offspring were a blend of their parents. This idea, known as blended inheritance, was quite a problem for natural selection because it would dilute favorable adaptations, limiting the power of selection to cause evolutionary change. Little did Darwin know that an Augustinian monk named Gregor Mendel was doing experiments with pea plants that would refute the ideas of blending inheritance and lay the foundation for modern genetics. Mendel bred more than 28,000 pea plants and very carefully mapped how traits like flower color and position, pea color and shape, and plant height passed from generation to generation. He discovered that traits were inherited discretely, though units ultimately