The story of where we come from evolves almost every year. It's a question many people ask, and the answer is getting more complicated as new evidence is emerging all the time.
For most of recorded history humankind has been placed on a metaphorical, and sometimes literal, pedestal. Sure, modern humans were flesh and blood like other animals.
But they were regarded as being so special that in the Linnaean taxonomy that prevailed well into the second half of the 20th century they were given their own family, the Hominidae.
This distinguished them from the Pongidae, the separate family used for the three African great apes – the common chimpanzee, bonobo and gorilla – plus the orangutan from Southeast Asia.
We now realise that modern humans are just one of the African great apes.
So when and how did this radically changed perception come about?
In the 19th century the only evidence available for determining the closeness of the relationship between any two living animals was how similar they were in terms of what the naked eye could tell from their bones, teeth, muscles and organs.
The first person to undertake a systematic comparative review of these differences between modern humans and the apes was English biologist Thomas Henry Huxley.
In the central section of a small book he published in 1863, called Evidence as to Man’s Place in Nature, Huxley concluded that the differences between modern humans and African apes were less than those between African apes and orangutans.
He speculated that because African apes were morphologically closer to modern humans than the apes from Asia, then the ancestors of modern humans were more likely to be found in Africa than elsewhere.
A closer inspection
Developments in biochemistry and immunology during the first half of the 20th century enabled the search for evidence of the relationships between modern humans and the apes to shift from macroscopic morphology to the morphology of molecules.
The results of applying a new generation of analytical methods to proteins were reported by the Austrian-born French biologist Emile Zuckerkandl and American biologist Morris Goodman in the early 1960s.
Zuckerkandl used enzymes to break up the protein component of haemoglobin into its peptide components. He showed that the patterns of the peptides from modern humans, gorilla and chimpanzee were indistinguishable.
Goodman used a different method, immunodiffusion, to study albumin, a serum protein. He showed that the patterns produced by the albumins of modern humans and the chimpanzee were identical. He concluded that this was because the albumin molecules were, to all intents and purposes, identical.
Apes and humans: related
Proteins are made up of a string of amino acids and in many instances one amino acid can be substituted for another without changing the function of the protein.
In the late 1960s, the American anthropologist Vince Sarich and New Zealand biologist Allan Wilson exploited these minor differences in protein structure and concluded that modern humans and the African apes were very closely related.
They also provided the first molecular clock estimate of modern human-African ape divergence, dating the split to only around five million years ago. This date was less than half of contemporary estimates based on fossil evidence.
In 1975 the American human geneticist Mary-Claire King and Allan Wilson showed that 99% of the amino-acid sequences of chimpanzee and modern human blood proteins were identical.
The discovery by James Watson and Francis Crick, with unwitting help from Rosalind Franklin, of the basic structure of DNA, and the subsequent discovery by Crick and others of the nature of the genetic code, meant that the relationships among organisms could be pursued at the level of the genome.
Nowadays technological advances mean that whole genomes can be sequenced. Over the past decade researchers have published good draft sequences of the nuclear genomes of the chimpanzee, orangutan, gorilla and the bonobo.