How Genetic Siblings Can Look So Very Different
Chromosomes are the building blocks of every living creature. Each human has 46 chromosomes -- 23 from his or her mother and 23 from his or her father. Chromosomes are made of protein and a single molecule of DNA.
Humans Vs. Neanderthals: How Did We Win?
Aug. 9, 2011 --
Up until about 30,000 years ago, humans shared the planet with Neanderthals, a relative so close to humans that our species interbred. In fact, some Neanderthal lives on in some of our DNA to this day. But around then, Homo sapiens were already well into the process of displacing Neanderthals, an undertaking that had been some 20,000 to 40,000 years in the making. How humans outpaced their relatives remains a mystery, but fossil evidence has left some clues about the scenarios that may have led to the downfall of Neanderthals. No single smoking gun is likely responsible for the disappearance of Homo neanderthalensis. Here, we explore some of the factors that likely contributed to their decline.
Field Museum Library/Getty Images
In the end, Neanderthals may have been wiped out because they simply lost the numbers game. As Homo sapiens moved from Africa into areas of southern Europe where Neanderthals had already been settled, the two species were placed in direct competition with one another. Eventually outnumbered 10 to one, Neanderthals were pushed to less favorable areas where food and shelter were more difficult to find, according to a study published last month in the journal Science. Resource competition and interbreeding wiped out the Neanderthals in this scenario.
Forced into Cannibalism?
With Homo sapiens pushing Neanderthals to fringe settlements, it’s possible that resource competition between Neanderthal groups forced them to turn to cannibalism. Fossil evidence suggests that may have been the case. Bones discovered in a cave in France show a group of Neanderthals defleshed the bones of others within their species for sustenance. They even ate humans. As grisly as the practice was, cannibalism also took a hidden toll on those who hunted and consumed their own species: a fatal epidemic similar to mad cow disease that caused severe mental impairments and wiped out thousands. These series of events could have contributed to the disappearance of Homo neanderthalensis.
The Fitter Specimen
In a battle of the brawn, Neanderthals would surely come out ahead. But in a footrace over a long distance, humans had the advantage. Humans were built for long-distance running, which allowed for hunting in hotter climates. Neanderthals, on the other hand, were strong and sturdy. They could run faster than humans, but only over a short sprint. As such, Homo neanderthalensis was better equipped for cooler climates. Distance-running and endurance could have given prehistoric Homo sapiens an edge when they entered Neanderthal strongholds in Asia and Europe, and came into direct competition with their cousins.
The Big Bang Theory
Neanderthals may not have quietly faded away so much as they went out with a bang, according to a study published last September in Current Anthropology. Around 40,000 years ago, a sequence of three major volcanic eruptions devastated Neanderthal homelands in Europe and Asia, speeding the demise of this species. Homo sapiens, by contrast, lived on the fringes beyond the range of the volcanic ash clouds. In other words, simple geographic luck could have led early humans to overtake Neanderthals.
Neaderthals had brawn, but early humans had a leg up on brains. Starting at birth, human and Neanderthal brains are similar. During the first year of life, however, the human brain begins more activity in neural circuitry. Although this doesn't mean that Neanderthals weren't as intelligent as humans, the brains of Homo sapiens developed to support higher-order functions, such as creativity and communication. Traces of Neanderthal creativity have been found, but no evidence has yet emerged to show they had a complex language of their own. However, according to one study published in the journal Medical Hypotheses, this lack of cognitive complexity also may have meant that Neanderthals didn't suffer from the same mental disorders as humans. This distinction, however, proved to be a net gain for humans and may have "helped early Homo sapiens survive in the process of natural selection," according to one report.
Humans Weren't to Blame
Neanderthals and humans were not in direct competition for too long, because Neanderthals disappeared earlier than once thought, according to one study published in May of this year in the Proceedings of the National Academy of Sciences, In this scenario, Neanderthals disappeared around 39,700 years ago -- 10,000 years earlier than is commonly believed. Since Homo sapiens arrived in the northern Caucasus region a few hundred years earlier, that didn't leave too much time for the two species to interact. This theory discounts any human intervention in the decline of Neanderthal populations, but still leaves open the possibility of other extinction scenarios.
Recent stories in the news about blond children being found in dark-haired and dusky-skinned Romany families -- children that weren't biological offspring -- prompted us to ask: Is it possible for two children from the same family to look completely different? It turns out the answer is, yes, it is possible. Here's how.
Nature is absolutely amazing when it comes to reproduction. There is a system specifically designed to make children different.
A person has a set of chromosomes. Each chromosome contains two halves that join in the middle so they look like an "X." All that a chromosome is, by the way, is two very tightly coiled strands of DNA (deoxyribonucleic acid).
The two halves of each chromosome come from the two parents. One half of the "X" in each chromosome comes from the mother, and the other half comes from the father. The two halves are bonded together at the middle of the "X." Each half of the "X" contains a complete set of genes, so each chromosome has two copies of every gene -- the "dominant" gene of the two is the one that is expressed.
A sperm and an egg meet to create a new person. The sperm carries one half of the new person's ultimate chromosomes, and the egg contains the other half.
The question then comes down to, "Where does the DNA in the sperm and the egg come from?" Every cell in the father contains a complete set of X-shaped chromosomes that are exactly alike. Same for the mother. The father's chromosomes came from his mother and father, and the mother's from her mother and father. To make a sperm cell, only one half of the X is contributed. But which half? This is where nature does the especially amazing part. When forming sperm cells, the father's body randomly chooses genes from the two halves of the father's chromosomes. This means that every sperm cell contains a random mix of the father's parents' genes. The same thing happens when forming eggs. Therefore, each child that a couple produces is a random mix of the four grandparents' genes.