Human beings , are bipedal primates belonging to the mammalian species Homo sapiens (Latin: "wise man" or "knowing man") in the family Hominidae (the great apes). Compared to other living organisms on Earth, humans have a highly developed brain capable of abstract reasoning, language, and introspection. This mental capability, combined with an erect body carriage that frees their upper limbs for manipulating objects, has allowed humans to make far greater use of tools than any other species. Humans originated in Africa about 200,000 years ago, but they now inhabit every continent, with a total population of over 6.6 billion as of 2007.

Like most primates, humans are social by nature; however, humans are particularly adept at utilizing systems of communication for self-expression, the exchange of ideas, and organization. Humans create complex social structures composed of cooperating and competing groups, ranging in scale from small families and partnerships to species-wide political, scientific and economic unions. Social interactions between humans have also established an extremely wide variety of traditions, rituals, ethics, values, social norms, and laws which form the basis of human society. Humans also have a marked appreciation for beauty and aesthetics which, combined with the human desire for self-expression, has led to cultural innovations such as art, literature and music.

Humans are also noted for their desire to understand and influence the world around them, seeking to explain and manipulate natural phenomena through science, philosophy, mythology and religion. This natural curiosity has led to the development of advanced tools and skills; humans are the only known species to build fires, cook their food, clothe themselves, and use numerous other technologies.

Most researchers currently accept the statement that "modern" humans can be considered to date to approximately 200-250 kyr. Others (such as Milford Wolpoff), take the view that our species extends as far as approximately 2.0 myr, subsuming H. erectus , H. ergaste r, and H. heidelbergensis . There are two polarizing camps on the issue of our species origin (though there is varying degrees of compromise between the two stances as well as various alternative positions): the multiregional (or continuity) camp, and the Out of Africa (replacement) camp.

The perspective of the Out of Africa model (often called Out of Africa II, referring to a second migration from Africa of a hominid population) adherents is that when there was a migration of H. erectus out of Africa into Asia and Europe, these populations (seen in materials like the Chinese and Indonesian erectus ) did not contribute a significant amount of genetic material to later populations that led to modern humans (some claim no genetic ancestry to these groups and their descendants at all, a "strict" replacement model). At approximately 200 kya there was a second migration of hominids out of Africa . This time it was fully modern H. sapiens , which proceeded to replace whatever populations that then occupied Asia and Europe . Some see direct competition and extermination of the native populations, some see passive replacement due to better adaptive strategies, and some see genetic admixture with the preponderance of genetic material coming from the incoming human populations, eventually replacing and assimilating them into the greater collective. In this view there is a specific speciation event that occurred which led to the origin of H. sapiens in Africa , and this population is the forerunner of modern humans, leaving the European Neanderthals, Chinese erectus , and others out in the cold.

There are various models which embody combinations of these ideas, different "strict" interpretations of the two theories, etc. Multiregionalists look for similarities between populations in the same geographic location that are separated spatially, while people who follow replacement look for differences. It is oft a difference of semantics between different interpretations rather than real differences of opinion, but often there is real disagreement on the validity of research, and theoretical interpretations. This has led to some fairly severe strife within the paleoanthropological community, with potshots often taken unfairly at rival theories and rival theoreticians. For example, multiregionalism is often portrayed as a racist theory that claims different "races" have evolved to different "levels" of intelligence. Out of Africa II has often been portrayed as a religiously motivated idea that tries to come to terms with the biblical story of Genesis, as reference to the "Eve" theory suggests.

Beyond disagreement over fundamental issues like "What is a valid speciation event?" one fact stands out: neither theory has proved itself above the other in terms of parsimonious explanation of the fossil evidence. The general opinion among researchers seems to go in cycles, supporting OoA, then supporting MRE, then supporting OoA, etc. Currently, we seem to be at a cusp of support for replacement, and there seems top be a shifting in opinion more favorable to continuity. The highly publicized genetic studies that purportedly "proved" that Neanderthals did not contribute the modern human genome are so plagued with practical and theoretical problems to make their conclusions moot, especially since it does not in any way address the rest of the populations in the world, and their genetic fate.

Humans are an eukaryotic species. Each diploid cell has two sets of 23 chromosomes, each set received from one parent. There are 22 pairs of autosomes and one pair of sex chromosomes. By present estimates, humans have approximately 20,000 – 25,000 genes. Like other mammals, humans have an XY sex-determination system, so that females have the sex chromosomes XX and males have XY. The X chromosome is larger and carries many genes not on the Y chromosome, which means that recessive diseases associated with X-linked genes, such as hemophilia, affect men more often than women.

Role of p53 gene in homo sapiens

Acts as a tumor suppressor in many tumor types; induces growth arrest or apoptosis depending on the physiological circumstances and cell type. Involved in cell cycle regulation as a trans-activator that acts to negatively regulate cell division by controlling a set of genes required for this process. One of the activated genes is an inhibitor of cyclin-dependent kinases. Apoptosis induction seems to be mediated either by stimulation of BAX and FAS antigen expression, or by repression of Bcl-2 expression.

Mutations of the gene encoding p53, a 53-kilodalton cellular protein, are found frequently in human tumor cells, suggesting a crucial role for this gene in human oncogenesis. To model the stepwise mutation or loss of both p53 alleles during tumorigenesis, a human osteosarcoma cell line, Saos-2, was used that completely lacked endogenous p53. Single copies of exogenous p53 genes were then introduced by infecting cells with recombinant retroviruses containing either point-mutated or wild-type versions of the p53 cDNA sequence. Expression of wild-type p53 suppressed the neoplastic phenotype of Saos-2 cells, whereas expression of mutated p53 conferred a limited growth advantage to cells inthe absence of wild-type p53. Wild-type p53 was phenotypically dominant to mutated p53 in a two-allele configuration. These results suggest that, as with the retinoblastoma gene, mutation of both alleles of the p53 gene is essential for its role in oncogenesis.