What makes us human?

style="float: right; margin-bottom: 10px; font-weight: 600;"Thu 25th Oct, 2012

When the entire human genome was sequenced eight years ago, the news came as a shock to many of us, not because of what scientists found, but because of what they didn't find. Most scientists had assumed that a sophisticated complex creature like a human would have 100,000 genes. They were wrong. It turns out we have just over 20,000 genes, about as much as a simple roundworm. This finding begged new questions, suc as if we only have 20,000 genes, then what is it that makes us different from a roundworm? What else besides genes could be influencing human evolution? Are we really that special on earth, after all?

Recently, scientists have found that genes, those pieces of DNA that carry our inherited traits, aren't the only way we pass on characteristics to our offspring. More than 90 percent of most genomes (including human) consists of DNA considered "junk." This DNA, which does not create proteins, had unknown functions or was considered a kind of molecular fossil record of our past. Later on it was found that this "junk" DNA serves a very important purpose. It regulates how genes make proteins, even preventing some proteins from being made. This so called "junk DNA" can be influenced by the environment, and field of study linking environment and DNA is known as epigenetics, and it has made a number of significant discoveries regarding how our genome is not only involved in making proteins, but also in telling our genes when and how.

But what does epigenetics has to do with us?
A 2011 study from Stanford University found that epigenetics may be what is separating us from our closest animal relatives, the chimpanzee. It certainly isn't our genome, we share at least 96 percent of our DNA with chimpanzees. Stanford researcher Dr. David Kingsley and his colleagues found that out of the three billion combinations of DNA bases that make up our genes, about 510 are present in chimps, but missing in humans. Many of these missing genes performed regulatory functions that are not found in humans anymore.

"If you alter the way a gene turns on or off at a particular stage in development, that can have a very large effect on a particular structure, but still preserve the other functions of a gene," said Dr. Kingsley.

In fact, Dr. Kingsley found two key areas that weren't regulated in humans (but still are in chimps). First, there is this one strip of DNA that sits near a gene responsible for a male hormone receptor. If the regulatory DNA is there, then the animal grows sensory whiskers and has altered male genitalia. Without this DNA strip, humans were free from growing whiskers, and developed traits that allowed for monogamous bonding. The other DNA strip Dr. Kingsley found sat near a gene called GADD45g, which inhibits cell growth in the brain. Humans don't have this strip of DNA, and thus were freed to eventually grow larger brains, Dr. Kingsley said. How could these differences have arisen?

Another group of researchers at the Georgia Institute of Technology, USA, found that these changes can arise from random insertions and deletions of tiny bits of DNA, which can eventually shut down certainly regulatory functions of genes that lead to the evolution of larger brains, less facial hair and other traits that make us human. The Georgia researchers found 68,000 of such alterations, indicating a fertile field for making changes outside our "genes," but within our DNA.

The Neanderthal Within
Archaeologists have long assumed that modern humans adopted their modern shape and behavior and split off from other humanoid primates about 200,000 years ago. The last competing group, the Neanderthals (named after the valley in Rhine-Westphalia where the first fossil was found), were assumed to have died off about 100,000 years ago. However, modern genetics analyses that can very quickly (and very cheaply) determine the sequence of an entire genome, have started to show another pattern. Two geneticists from the University of Pennsylvania, USA, reported on July 31 in the journal Cell, that Neanderthals may have lived alongside humans as early as 25,000 years ago. They also suggested that modern humans and Neanderthals interbred in Africa, Asia and Europe. The researchers made this discovery by sequencing the genomes of three hunter-gatherer societies in Tanzania and Cameroon. Their genomes carry unusual DNA sequences in the Africans tribes, which suggested that Neanderthal and human interbreeding occurred much later than previously thought, and that Neanderthals were much closer to being a human species than thought.

It's a Question of Culture, after all
All these findings about our genes, evolution and physiology boil down to one key aspect: culture. And culture, too, may not be exclusive of humans. A research team headed by two paleontologists at the University of Witwatersrand in South Africa found fossil evidence in that country showing that modern culture goes back at least 44,000 years (and not 10,000 to 20,000, as many assumed). This date, of course, puts the rise of human culture at about the same time when we were intermingling with Neanderthals. The team reported online on July 30 in the journal Proceedings of the National Academy of Sciences, that they had found wooden beating sticks, jewelry made from ostrich eggs and marine shells, fine bone points to use as awls and poison-tipped arrowheads. The items, found at Border Cave in South Africa, represent a unique finding. But "many elements of material culture that characterize the life style of (native) hunter-gatherers in Southern Africa were part of the culture...44,000 years ago," said research head Lucinda Blackwell.
So, all in all it seems are us, humans are the results of genes influenced by nature, ancestors mingling with other species, and a culture that was probably shared.

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