Cellular respiration is the process most cells use to convert food molecules into energy. In multicellular organisms like humans and trees, cellular respiration takes place in the mitochondria. These important organelles and the high-energy molecules of ATP they produce power virtually every biochemical reaction that takes place -- both in your body and in the plants and animals around you. While scientists have long understood the importance of mitochondria in energy production, it wasn't until the early 1990s that geneticists began to recognize that mitochondria might help explain the path of human evolution.

Efforts to understand human evolution have been fraught with controversy ever since the study of human and humanlike fossils began. One of the most enduring and heated debates to come out of the field of paleoanthropology has to do with whether Neanderthals ever interbred with early Homo sapiens. The fossil record shows that Neanderthals roamed the earth, in and around what is now modern-day Europe, until about 30,000 years ago, which means they and early Homo sapiens coexisted. But were these two groups of hominids (as scientists call all humanlike creatures) close enough culturally and, more important, genetically to have interbred?

One possible answer to this question has come from scientists studying mitochondria. These energy-producing organelles contain their own DNA distinct from the DNA found in the nuclei of most of our cells. Individual cells may have hundreds of mitochondria, which means that each cell will also have hundreds of copies of mitochondrial DNA (mtDNA). This is important given that DNA in fossils breaks down over time. After 30,000 years, fossilized Neanderthal cells have only fragments of DNA left in them -- not enough to piece together the entire Neanderthal genome but sometimes enough to assemble a complete Neanderthal mtDNA genome.

As with nuclear DNA, the genetic sequence of mtDNA evolves over time. Some scientists suggest that the rate of change -- the mutation rate -- is fairly constant. If this is the case, then the amount of difference between the genetic sequences of two fossil individuals should be a good measure of the amount of time that has passed since the genetic lineages of the two individuals diverged.

An analysis of mtDNA taken from two Neanderthal fossils and from hundreds of contemporary humans showed a great deal of difference between the two groups. While each group showed variability among individuals within the group, there was three times as much variability between two individuals of different groups. The scientists who conducted the research say the amount of variability they observed between modern Homo sapiens and Neanderthals could not have arisen in just 30,000 years. They conclude that Homo sapiens and Neanderthals must have diverged hundreds of thousands of years ago and so could not have interbred. Critics argue that until more evidence is found, the relationship between humans and Neanderthals remains an open question.