Haeckel, Ernst: A German biologist who lived from 1834-1919, Haeckel was the first to divide animals into protozoan (unicellular) and metazoan (multicellular) forms. His notion of recapitulation is no longer accepted.
Haile Selassie, Yohannes: A paleoanthropologist who, while doing field work in Ethiopia for his doctoral dissertation at the University of California, Berkeley, discovered Ardipithecus ramidus kadabba, a bipedal hominid dated at5.2 million years old.
half-life: The amount of time it takes for one-half of the atoms of a radioactive material to decay to a stable form. For example, the half-life ofcarbon-14 is 5,568 years.
Hamilton, W.D.: A naturalist, explorer, and zoologist who worked in the world of mathematical models, including "Hamilton's Rule," about the spread through a population of a gene for altruistic self sacrifice. He was also interested in the evolutionary impact of parasites as the key to many outstanding problemsl eft by Darwin, including the baffling riddle of the evolution of sex. This led him to extensive work in computer simulations.
haploid: The condition of having only one set of genes or chromosomes. In normally diploid organisms such as humans, only the gametes are haploid.
haplotype: A set of genes at more than one locus inherited by an individual from one of its parents. It is the multi-locus analog of an allele.
Hardy-Weinberg principle: In population genetics, the idea that if a population experienced no selection, no mutation, no migration, no genetic drift, and random mating, then the frequency of each allele and the frequencies of genotype in the population would remain the same from one generation to the next.
Hardy-Weinberg ratio: The ratio of genotype frequencies that evolve when mating is random and neither selection nor drift are operating. For two alleles (A and a) with frequencies p and q, there are three genotypes: AA, Aa, and aa. The Hardy-Weinberg ratio for the three is: p2AA : 2pqAa : q2aa. It is the starting point for much of the theory of population genetics.
Harvey, Ralph: A geologist whose work includes the study of geological processes at a range of scales, from the smallest nanometer to broader-scale interpretations of the history experienced by geological materials.
heavy metals: Metals with a high relative atomic mass, such as lead, copper, zinc, and mercury. Many of them are toxic.
hemoglobin: A protein that carries oxygen from the lungs throughout the body.
heredity: The process by which characteristics are passed from one generation to the next.
heritability: Broadly, the proportion of variation (more strictly, variance) in a phenotypic character in a population that is due to individual differences in genotypes. Narrowly, it is defined as the proportion of variation (more strictly, variance) in a phenotypic character in a population that is due to individual genetic differences that will be inherited in the offspring.
heritable: Partly or wholly determined by genes; capable of being passed from an individual to its offspring.
Herrnstein, Richard J.: A professor of psychology and an author of notable books on intelligence and crime. He has primarily done research on human and animal motivational and learning processes. His books include Psychology and I. Q. in the Meritocracy, and he coauthored (with Charles Murray) The Bell Curve: Intelligence and Class Structure in American Life (1994).
heterogametic: The sex with two different sex chromosomes (males in mammals ,because they are XY). Compare with homogametic.
heterozygosity: (for most purposes) The proportion of individuals in a population that are heterozygotes.
heterozygote: An individual having two different alleles at a genetic locus. Compare with homozygote.
heterozygote advantage: A condition in which the fitness of a heterozygote is higher than the fitness of either homozygote.
heterozygous: Having two different alleles for a particular trait. See also heterozygote.
Hill, Andrew: A paleontologist and professor at Yale University. His work with Mary Leakey's team at Laeotoli, Tanzania, in the 1970s helped lead to the discovery of the fossilized footprints of early hominids and other mammals. His current research interests include hominid evolution, paleoecology, and taphonomy.
HIV: Human Immunodeficiency Virus. The virus causes AIDS by inactivating the Tcells of the immune system.
homeobox: Homeoboxes are relatively short (approximately 180 base pair)sequences of DNA, characteristic of some homeotic genes (which play a central role in controlling body development). Homeoboxes code for a protein "homeodomain," a protein domain that binds to DNA, and can regulate the expression of other genes. These homeodomain motifs are involved in orchestrating the development of a wide range of organisms.
homeobox genes: "Homeobox genes" are genes that contain "homeoboxes", and can regulate the expression of other genes. There are at least 24 homeobox genes, some but not all of which are also homeotic in their effect. In general, “homeotic" genes are genes that control the identity of body parts. They are active in the early stages of embryonic development of organisms. Some, but not all, homeotic genes are homeobox genes. "Hox" genes are a subgroup of homeotichomeobox genes that determine positional cell differentiation and development. They lay out the head to tail body pattern in very early embryos. The Hox gene sare very ancient and widely shared among bilateral animals. After the head to tail pattern is established, homeotic genes direct the developmental fates of particular groups of cells.
homeostasis (developmental): A self-regulating process in development, such that the organism grows up to have much the same form independent of the external influences it experiences while growing up.
homeotic mutation: A mutation causing one structure of an organism to grow in the place appropriate to another. For example, in the mutation called "antennapedia" in the fruit fly, a foot grows in the antennal socket.
hominids: Members of the family Hominidae, which includes only modern humans and their ancestors since the human lineage split from the apes.
homogametic: The sex with two of the same kind of chromosomes (females in mammals, because they are XX). Compare with heterogametic.
homologous structures: The structures shared by a set of related species because they have been inherited, with or without modification, from their common ancestor. For example, the bones that support a bat's wing are similar to those of a human arm.
homology: A character shared by a set of species and present in their common ancestor. Compare with analogy. (Some molecular biologists, when comparing two sequences, call the corresponding sites "homologous" if they have the same nucleotide, regardless of whether the similarity is evolutionarily shared from a common ancestor or convergent. They likewise talk about percent homology between the two sequences. Homology in this context simply means similarity. This usage is frowned upon by many evolutionary biologists, but is established in much of the molecular literature.)
homozygote: An individual having two copies of the same allele at a genetic locus. Also sometimes applied to larger genetic entities, such as a whole chromosome; a homozygote is then an individual having two copies of the same chromosome.
homozygous: Having identical alleles for a particular trait. See also homozygote.
Homo erectus: A species of hominid that lived between 1.8 mya and 300,000 years ago; the first Homo species to migrate beyond Africa.
Homo habilis: A species of hominid that lived between 1.9 and 1.8 mya, the first species in genus Homo, and the first hominid associated with clear evidence of tool manufacture and use.
Homo neanderthalensis: A species of hominid that lived between 150,000 and30,000 years ago in Europe and Western Asia, originally thought to be a geographic variant of Homo sapiens but now generally accepted to be a distinct species.
Homo sapiens: Modern humans, which evolved to their present form about 100,000years ago.
horsetail: A seedless plant related to ferns. Twenty-five species of only one genus, Equisteum, remain today, whereas many different species, some the size of modern trees, were abundant in ancient swamps. Along with lycophytes and ferns, horsetails were among the first terrestrial plants to appear.
Ho, David: A physician and world-renowed AIDS researcher. Dr. Ho overturned an earlier conventional assumption that the HIV virus remains dormant for up to 10 years in a person before its outbreak into AIDS. His recognition that the virus is active right from the beginning of infection led him to initiate the deployment of a combination of drugs to overpower the virus.hox genes: A particular subgroup of homeobox genes that function to pattern the axis of an organism Õs body and determine where limbs and other body segments will grow as the embryo develops.
Huxley, Thomas Henry: British intellect, photographer, and contemporary of Darwin. He was the first to apply the theory of natural selection to humanity to explain the course of human evolution.
hybrid: The offspring of a cross between two species.
hypothesis: An explanation of one or more phenomena in nature that can be tested by observations, experiments, or both. In order to be considered scientific, a hypothesis must be falsifiable, which means that it can be proven to be incorrect.