Gene Flow and Migration

Another source of genetic change in human populations is gene flow, the exchange of genes between populations. Gene flow occurs directly when individuals from one population mate with members of another population, thereby introducing their genes into the population. Increased gene flow between populations generally makes them more alike than they had been previously. Gene flow also occurs indirectly. For example, if population A interbreeds with population B, and population B interbreeds with population C, some genes from population A will pass to population C. In this way, gene flow occurs across vast geographic regions and connects distant populations. In fact, global gene flow maintains the unity of the human species, ensuring that people from any two populations in the world can successfully mate. If a human population became isolated and no longer shared gene flow with other populations, it might, over hundreds of thousands of years, lose the ability to breed successfully with other human populations. At that point the isolated population would be considered a new species.
In humans, gene flow often occurs as a result of migration. Migrations most frequently occur on a small scale, as when individuals or families move to a neighboring village, town, or city. Small-scale migration usually takes place at short distances and is reciprocal—that is, members of neighboring populations each migrate to the region of the other population. Large-scale or mass migrations occur when a large group of people moves to a new region, often because of the effects of war or natural disaster.
Mass migration and major population resettlements dramatically increase gene flow. For example, Africans who were brought to the United States as slaves, as well as their descendants, intermixed with white populations. Today the gene pool of those who identify themselves as African American is intermediate between that of American whites and African blacks. On average, African Americans in the United States have 30 percent European ancestry. Those African Americans in the northern United States may have up to 50 percent European ancestry while those in the Southern states—where laws and cultural values long prohibited racial mixing—may have as little as 10 percent European ancestry. This difference illustrates the power that psychological and cultural barriers can have in decreasing gene flow. People who feel deeply rooted in a particular racial or ethnic group may have some animosity toward the mating of people with different physical appearances or from different cultural backgrounds. Religious and socioeconomic differences can also act as barriers to gene flow. However, people are highly social by nature. Even with the effects of racism and ethnocentrism (a belief in the superiority of one’s own social or cultural group), people have always intermarried and interbred with members of neighboring groups.
Historically, natural barriers such as large rivers, seas, deserts, and mountain ranges have prevented migration and reduced gene flow between certain regions. Geographic distance also impeded migrations; people preferred to migrate only short distances. Over the course of the past several centuries, technological improvements in transportation have reduced the influence of geography and distance. For instance, people now can travel easily from one side of the world to the other within a day by airplane. In general, however, populations tend to be more similar to their neighbors and more different from populations that live far away.

Human origin

The differences among modern human populations developed in the evolutionary past. Scientists believe that humans evolved from apelike ancestors beginning about 5 million years ago. The predecessor of modern humans, Homo erectus, lived in Africa and migrated to Asia and Europe 1 million to 2 million years ago. Scientists generally agree that anatomically modern humans, Homo sapiens, evolved within the last 200,000 years. However, anthropologists disagree about how and where modern humans evolved. There are two major hypotheses about how modern humans evolved: the out of Africa hypothesis and the multiregional hypothesis.
According to the out of Africa hypothesis, modern humans originated in Africa in the last 200,000 years and spread from there to the rest of the world, including the Americas and Australia. This migration out of Africa to the rest of the world took place within the last 100,000 years and may have begun as recently as 50,000 to 70,000 years ago. Based on this hypothesis, the differences among modern humans today originated relatively recently—mostly after the great dispersal out of Africa, although some differences may have formed in Africa. According to the competing multiregional hypothesis, modern humans developed in parallel in Africa, Europe, and Asia over 1 million or 2 million years from existing populations of Homo erectus. In this scenario, differences between human populations originated in the distant past. The original support for the multiregional hypothesis derived from fossil evidence that suggested continuity of evolution between archaic humans in Europe, known as Neandertals, and modern Europeans. Certain fossils suggested similar continuity between archaic and modern humans in East Asia. The out of Africa hypothesis was first proposed based on genetic studies of a type of DNA known as mitochondrial DNA, which is inherited through the maternal line. Since then, studies of the Y chromosome, which is inherited through the paternal line, have confirmed the results of mitochondrial DNA studies. These studies show that living African populations have more genetic diversity than any other human groups, and that this diversity has been accumulating for perhaps 100,000 to 200,000 years. This finding implies that all modern humans are descended from a small population of Homo sapiens that lived in Africa 100,000 to 200,000 years ago. Analysis of mitochondrial DNA from a Neandertal fossil found in Germany also suggests that Neandertals did not contribute DNA to modern Europeans. Thus, evidence has been accumulating that modern humans are not descended from Neandertals living outside of Africa. Today, many geneticists and physical anthropologists see the balance of the evidence as strongly favoring the out of Africa hypothesis. For more information on the evolution of modern humans we can also find about Human Evolution in: "Theories of Modern Human Origins and Diversity".
Another important finding is that human genetic variation between groups, however defined, is small compared to that within groups. The data strongly support the idea that all living humans originated recently from a relatively small population—on the order of thousands or tens of thousands of individuals. All people share a strong genetic heritage, and are much more alike.

Embryology

Embryology is the branch of biology dealing with the development of the animal embryo. (For the embryology of plants, see Fertilization; Plant; Seed.) Embryology includes within its province the development of the fertilized egg and embryo and the growth of the fetus.
Until the second half of the 18th century, embryology was a matter of speculation rather than of knowledge. One generally accepted theory was that of preformation: The complete animal with all its organs was believed to exist in the germ in miniature, needing only to unfold like a flower. It followed that each germ must contain within itself the germs of all its future descendants, one within another, as in a nest of boxes. Many naturalists believed the germ to be contained in the ovum, the female germ cell, but after the microscope had revealed spermatozoa, the male germ cells, in 1677, a school of so-called spermists advanced the hypothesis that the germ was contained in the spermatozoon. Their drawings show the spermatozoon encasing a minute human figure, called the homunculus.
Little attention was given to the theory, called the theory of epigenesis, that the English physician and anatomist William Harvey had stated in 1651. This theory, which had been vaguely expressed much earlier by Aristotle, held that the specialized structures of the individual develop step by step from unspecialized antecedents in the egg. Proof of this theory was not forthcoming, however, until 1759 when the German anatomist Kaspar Friedrich Wolff reported on his study of the development of the chick in the egg and showed that the organs arise from undifferentiated material. The basic potential nature and organization of the structures of the organism are determined by the genetic constitution of the fertilized egg (see Heredity). Wolff is called the founder of modern embryology, a title also sometimes given to the Estonian naturalist Karl Ernst von Baer, who in the 19th century described the principal phases in the development of the chick and pioneered in comparative embryology.

Paleoanthropology

The scientific study of human evolution is called paleoanthropology. Paleoanthropology is a subfield of anthropology, the study of human culture, society, and biology. Paleoanthropologists search for the roots of human physical traits and behavior. They seek to discover how evolution has shaped the potentials, tendencies, and limitations of all people. For many people, paleoanthropology is an exciting scientific field because it illuminates the origins of the defining traits of the human species, as well as the fundamental connections between humans and other living organisms on Earth. Scientists have abundant evidence of human evolution from fossils, artifacts, and genetic studies. However, some people find the concept of human evolution troubling because it can seem to conflict with religious and other traditional beliefs about how people, other living things, and the world came to be. Yet many people have come to reconcile such beliefs with the scientific evidence.
All species of organisms originate through the process of biological evolution. In this process, new species arise from a series of natural changes. In animals that reproduce sexually, including humans, the term species refers to a group whose adult members regularly interbreed, resulting in fertile offspring—that is, offspring themselves capable of reproducing. Scientists classify each species with a unique, two-part scientific name. In this system, modern humans are classified as Homo sapiens.

Hermaphroditism

In some relatively simple animals such as the earthworms and leeches, organs producing sperm and ova occur in the same individual (see Hermaphroditism). Although such animals produce both male and female gametes, the production of sperm and ova usually occurs at different times, so that these animals generally do not fertilize themselves but rather other individuals of the same species. Certain hermaphroditic animals, such as the planarian flatworms, habitually undergo self-fertilization. Among plants, one individual may bear reproductive organs of only one sex, separate reproductive organs of both sexes, or reproductive organs containing both male and female elements (see Flower). Individuals among higher animals bear reproductive organs of only one sex.

Metabolism (chemistry)

Metabolism (chemistry), inclusive term for the chemical reactions by which the cells of an organism transform energy, maintain their identity, and reproduce. All life forms—from single-celled algae to mammals—are dependent on many hundreds of simultaneous and precisely regulated metabolic reactions to support them from conception through growth and maturity to the final stages of death. Each of these reactions is triggered, controlled, and terminated by specific cell enzymes or catalysts, and each reaction is coordinated with the numerous other reactions throughout the organism.

All rights reserved.In keeping with the first two laws of thermodynamics, organisms can neither create nor destroy energy but can only transform it from one form to another. Thus, the chlorophyll of plants, at the foundation of almost all food and energy-transfer webs (see Food Web), captures energy from sunlight and uses it to power the synthesis of living plant cells from inorganic substances such as carbon dioxide, water, and ammonia. This energy, in the form of high-energy products (carbohydrates, fats, and proteins), is then ingested by herbivores and secondarily by carnivores, providing these animals with their only source of energy and cell-building chemicals.

Virtually all living organisms, therefore, ultimately derive their energy from the sun. On reproducing, each species member—whether green plant, herbivore, or carnivore—passes on specific genetic instructions on how to intercept, transform, and finally release energy back into the environment during its life span. Metabolism, from a thermodynamic point of view, embraces the processes by which cells chemically intercept and distribute energy as it continuously passes through the organism.

Bacteriology

Bacteriology, study of bacteria, including their classification and the prevention of diseases that arise from bacterial infection. The subject matter of bacteriology is distributed not only among bacteriologists but also among chemists, biochemists, geneticists, pathologists, immunologists, and public-health physicians. Bacteriology is part of the broader field of microbiology, the study of microorganisms.

Bacteria were first observed by the Dutch naturalist Antoni van Leeuwenhoek with the aid of a simple microscope of his own construction. He reported his discovery to the Royal Society of London in 1683, but the science of bacteriology was not firmly established until the middle of the 19th century. For nearly 200 years it was believed that bacteria are produced by spontaneous generation. The efforts of several generations of chemists and biologists were required to prove that bacteria, like all living organisms, arise only from other similar organisms. This fundamental fact was finally established in 1860 by the French scientist Louis Pasteur, who also discovered that fermentation and many infectious diseases are caused by bacteria. The first systematic classification of bacteria was published in 1872 by the German biologist Ferdinand J. Cohn, who placed them in the plant kingdom. They are now usually included in the kingdom Prokaryote. In 1876 Robert Koch, who had devised the method of inoculating bacteria directly into nutrient media as a means of studying them, found that a bacterium was the cause of the disease anthrax.

Since 1880, immunity against bacterial diseases has been systematically studied. In that year, Pasteur discovered by accident that Bacillus anthracis, cultivated at a temperature of 42° to 43° C (108° to 110° F), lost its virulence after a few generations. Later it was found that animals inoculated with these enfeebled bacteria showed resistance to the virulent bacilli. From this beginning date the prevention, modification, and treatment of disease by immunization, one of the most important modern medical advances. See Antitoxin.

Other significant developments in bacteriology were the discoveries of the organisms causing glanders (1862), relapsing fever (1868), typhoid fever (1880), tetanus (1885), tuberculosis (1890), plague (1894), bacillary dysentery (1898), syphilis (1905), and tularemia (1912).