A new study suggests that ancient and modern humans looked distinctive because of the diseases which eventually altered the genes of the group affected. Scientists reconstructed DNA maps to show how genes were altered and passed down through generation.

Researchers from the Hebrew University, led by Liran Carmel, reconstructed the DNA methylation maps of the Denisovans and the Neanderthals. DNA methylation is a biochemical process by which a methyl group binds with a DNA. Once the process alters the individual's genes, this change will manifest even on the later generation.

They examined the cytosines, one of the five bases found in nucleotides. They chose this base because this is where DNA methylation or alteration happens and measuring its decay rate will allow them to have a comprehensive picture of its history. They want to find out the possible external factors that led to the genetic alterations of the ancient humans.

Their analysis revealed that there are 2,000 DNA regions that vary between ancient and modern humans. One of these regions is the HoxD cluster-a DNA sequence that determines the physical appearance. This explains why Neanderthals have shorter and stouter compared to modern humans. As for the Denisovans, researchers cannot compare as they only have the fossils of a finger bone and two unusually large teeth as basis.

The researchers concluded that the ancient and modern humans' appearances are different because of the diseases that caused the DNA methylation. About 35 percent of the DNA variations seen on the samples were related to the brain and bone structure.

However, when asked if DNA methylation has something to do with mental disabilities such as schizophrenia, autism, and Alzheimer's present to modern humans, Carmel was uncertain if it began in ancient humans or just recently emerged.

"We see associations with diseases like schizophrenia, autism and Alzheimer's," said Carmel to The New Scientist. "Could it be that recent changes in the activity of genes in our brain also led to psychiatric disorders?"

Further details of this DNA analysis can be read on the April 17 issue of Science.