By Richard Black
BBC science correspondent in Seattle
New technologies may soon allow scientists to identify some of the genes of humankind's oldest ancestors.
Ancient molecules may give a better idea of what we once looked like
This raises the possibility of plotting the evolutionary tree of humanity from five million years ago to the present.
Professor Hendrik Poinar says DNA fragments should be recoverable from fossils that are a million years old, and proteins from even older times.
His comments came at the American Association for the Advancement of Science annual meeting in Seattle.
Professor Poinar, from McMaster University in Canada, said the key was to find fossils preserved so well that samples had not been degraded.
DNA, the biomolecule in cells that carries the "code of life", starts to fall apart rapidly under unfavourable conditions.
Proteins, on the other hand, which build and maintain the body and are constructed from information in the DNA, are more robust.
The oldest protein from a fossil analysed so far was extracted from a bison (Bison priscus) which died 55,000 years ago.
The animal died in Siberia, where the intense cold acted like a natural deep freeze.
A team led by Christina Nielsen-Marsh, of Newcastle University, UK, extracted and identified samples of a bone protein called osteocalcin.
But Professor Poinar, who works out of McMaster's department of anthropology, believes we can go back substantially further.
"DNA, for example; in theory it should survive for about 100,000 years," he told BBC News Online.
"In reality, the oldest reproducible sequences go back around 300,000, so already we're beyond theory.
"But I think small fragments could survive for a million years, in a cold climate."
In 2001, a claim was made for the recovery of 1.8-million-year-old DNA found in a specimen taken from South Africa's Sterkfontein Caves, famous for their remains of ancient human-like creatures. But it was a claim that was greeted with wide scepticism.
However, the more durable proteins, Professor Poinar believes, could survive in samples as old as five million years.
That would take researchers back in time to beyond even "Lucy", the Ethiopian hominid that lived 3.2 million years ago and is thought to have given rise to the Homo line which ended in modern humans.
From proteins, scientists can deduce genetic sequences which could be compared with equivalent genes in modern humans, enabling researchers to work out phylogenetic pathways and provide better information on what our ancient ancestors might really have looked like.
But where the sample is found and how it was preserved will be crucial.
"Realistically, permafrost and ice are the best places," Professor Poinar said. "If that is not possible, I would go for caves.
"You can go into caves and they can maintain constant conditions of temperature and humidity over long periods of time, even though the outside temperature may be way above what it is inside the cave."
After samples of a protein have been extracted, they must be analysed, using a relatively new technique known as time-of-flight mass spectrometry.
It is a slow but highly sensitive process that allows researchers to work out the protein's component parts. And from this information, scientists can reconstruct the genetic sequence that gave rise to the protein.
But Professor Poinar told the AAAS meeting that nothing could be accomplished without finding the right samples.
Relics of relatively modern human relatives such as Neanderthals have been found in caves. More samples of older hominids from similar sites would be most welcome, he said.