Up until about 1 billion years ago organisms produced asexually (or without sex) by dividing, which meant they basically copied their own genetics with little variations. When scientists began to see evidence of sexual reproduction is also when they began to notice much more complex organisms. This is also where evolution began to be present because with the beginning of genetic variation is the only way for evolutionary changes to be altered; due to a combination of two parent genes coming together to form an entirely different offspring. Most evolutionary change is believed to have arose only within the last 30 percent of life's history. This is attributed to genetic variation in several ways. The alleles (or variantes of a gene which code for different expressions of a trait) is what each individual has a pair of. In many cases the recessive (dominant) alleleis the one which holds physical ground appearing in an individuals phenotype. Although, there are also several relationships which are codominant where both allele are expressed in the phenotype. For example a humans ability to taste phenylthiocarbamide (PTC) is controlled by the T gene, the inability to taste this chemical (found in many plants) is represented as t. Therefore there are three possible combinations, TT, Tt, tt. The uppercase letter is always dominant, so, the first person would taste is,(there genotype would be aclled homozygous due to two of the same letters) the second would also(they would have a heterozygous genotype due to two different letters), but the third would not because there is no dominant ability in their genotype. This is why two pairs of allels are needed in order to maintain genetic variation. When an offspring is produced the gamete (sex cell) separate taking only one allele from each pair. .
Sex and Human Evolution.
Im most mammals sexual activity takes place only when reproducing and offspring.