The near-absence of recombination in mitochondrial DNA makes it useful for population genetics and evolutionary biology. If all the mitochondrial DNA is inherited as a single haploid unit, the relationships between mitochondrial DNA from different individuals can be seen as a gene tree. Patterns in these gene trees can be used to infer the evolutionary history of populations. The classic example of this is where the molecular clock can be used to give a date for the so-called mitochondrial Eve. This is often interpreted as strong support for the spread of modern humans out of Africa. Another human example is the sequencing of mitochondrial DNA from Neanderthal bones. The relatively large evolutionary distance between the mitochondrial DNA sequences of Neanderthals and living humans is evidence for a general lack of interbreeding between Neanderthals and anatomically-modern humans.