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Unearthing Neanderthal's Human Legacy
The Neanderthal Debate
When you think of Neanderthal man, you're likely to get some pretty iconic images of a primitive sub-human species with a small brain low sloping forehead and thick brute like limbs and torso similar to what we describe as Cro-Magnon man. But even Cro-Magnons are considered pretty much just like us. They take their name from a cave in France where Louis Lartet found them in 1868. Unlike Neanderthals, Cro-Magnons are not considered a separate species from Homo sapiens. In fact, they're the earliest known European example of our species—living between 35,000 and 10,000 years ago—and are actually modern in every anatomical respect. They did, however, have somewhat broader faces, a bit more muscle, and a slightly larger brain. The idea that Neanderthal is a separate species altogether started with the very discovery of their skeletal remains.
Recent new discoveries hope to shed light on the Neanderthal. Scientists have been able to scan the genomes of Neanderthals who died 50,000 years ago. And yet debate still rages - do Neanderthals belong within Homo sapiens or a separate species? It's a debate that extends beyond Neanderthals, forcing us to ask what it means to be a species at all. Paleoanthropologists cannot agree. However, they do agree that many of the stereotypes first assigned to Neanderthals are wrong. They had complex tools and language, made homes and farmed, buried their dead and raised families. In short they were much like us even contributing to our own genealogy.
Neanderthal Gets an Early Bad Rap
In August 1856 in the German valley of Neander (Neanderthal in German) men cutting limestone stumbled upon some bones in a cave. The bones—a piece of a skull, portions of limbs, and fragments of shoulder blades and ribs—eventually were shown to an anatomist in Bonn named Hermann Schaafhausen. He noticed that the bones had the overall shape you'd expect from a human skeleton, but with some strange features. The skullcap sported a heavy brow ridge hanging over the eyes like a boney pair of goggles. intrigued, Schaaffhausen compared the skulls to several other skulls with similar features that had been discovered in Belgium some years earlier in 1829. Schaaffhausen eventually realized the significance of this find - a potentially new human species - and published a paper on the Neanderthal fossils in 1858. He believed the Neanderthal fossils dated from the Glacial Period when extinct animals such as mammoths and the wooly rhinoceros still lived in Europe, which would make them among the oldest human remains known. This was before scientists believed humans lived during the Ice Age. Schaaffhausen argued that the skeleton belonged to a savage and barbarous race of humans that lived in Europe before modern peoples migrated there. The fossils generated considerable debate among anthropologists in Germany and abroad. The prominent German anthropologist Rudolf Virchow rejected Schaaffhausen's interpretation of the fossils, considering them the pathological remains of an ancient human. However, in 1864 William King, professor of geology at Queens College in Galway, Ireland, presented a paper where he argued the Neanderthal fossils belonged to an extinct species of early human that he named Homo neanderthalensis.
Expanding the Early Human Family
Around the time of the first Neanderthal remains being found, other fossils were beginning to be discovered that would help paleontologists piece together Neanderthal's place in the evolution to homo sapiens.
In 1891, Java, Indonesia The first Homo Erectus fossil, commonly known as Java Man, was discovered. By 1940 many more remains had been found there, and in China. The earliest examples lived about 1.8 million years ago in Africa and western Asia, and the species may have survived as late as 200,000 years ago in Indonesia. Homo erectus is the oldest known species to have a human-like body, with relatively elongated legs and shorter arms in comparison to its torso size. The first human species known to have spread out of Africa, Homo erectus was a tool-maker and omnivore, and perhaps the first to control fire.
In 1907, near Heidelberg,Germany, the jawbone of an extinct species of man was discovered. it was described and named by Otto Schoetensack as Homo Heidelbergensis (sometimes called Homo rhodesiensis). Since this discovery other Homo Heidelbergensis skeletons have been found dating to the same perod with 90 percent of the known skeletons coming from a discovery in 1992 at a site in the Sierra de Atapuerca in northern Spain. A Spanish team had located more than 5,500 human bones dated to an age of at least 350,000 years. It is thought Homo Heidelbergensis is an extinct species of the genus Homo which lived in Africa, Europe and western Asia from at least 600,000 years ago, and may date back 1,300,000 years predating Neanderthal. It survived until about 200,000 to 250,000 years ago. Its brain was nearly as large as that of a modern Homo sapiens.
In 1974, Ethiopia, the first Australopithecus afarensis, known as Lucy was discovered with around 40% of her full skeleton. It lived between about 3.8 and 3.0 million years ago in eastern Africa. Australopithecus afarensis is one of the oldest early human relatives that experts agree habitually walked on 2 legs (bipedalism). Older species such as Ardipithecus ramidus, living around 4.5-4.3 million years ago, also show signs of bipedalism according to some experts. A. afarensis is known from many fossil finds in Tanzania, Kenya and Ethiopia, but Lucy is particularly important because she is the most complete and well-preserved afarensis fossil ever found.
In 2003, island of Flores, Indonesia, was unearthed Homo floresiensis, the skeleton of a 1m-tall individual, nicknamed ‘the hobbit’, who lived between about 95,000 and 17,000 years ago. Homo floresiensis has been nicknamed 'the hobbit' because of its relatively small size – approximately 1-1.2 metres (3-4 feet) tall. Its brain was also of very small size, though some experts believe it was more human-like in shape.
In 2008, southern Siberia was unearthed a 40,000-year-old fossils named Denisovan (adult tooth and fossilized pinkie bone). Recently, scientists successfully extracted nuclear DNA from the pinkie bone and conducted comparison studies with the genomes of modern humans and Neanderthals. Studies show the girl was closely related to Neanderthals, yet distinct enough to merit classification as a new species of archaic humans. The Denisovan genome also suggests the young girl had brown hair, eyes, and skin. It appears this ancestral group contributed exclusively to populations in Southeast Asia and Oceania.
In 2009, in Ethiopia, was discovered an Ardipithecus ramidus fossil known as Ardi, The most complete ancient hominin skeleton 4.4 million years old. In fact so old that it may be the last common ancestor of humans and chimpanzees.
New Neanderthal DNA Evidence
Over the past 15 years, Svante Pääbo, a geneticist at the Max Planck Institute of Evolutionary Anthropology in Germany, and his colleagues began investigating the DNA evidence from Neanderthal remains. Starting with those fossils from the Neander Valley, they extracted bits of genetic material that had survived tens of thousands of years. Eventually, they were able to assemble the fragments into the entire Neanderthal genome. It's clearly different from the genome of any human alive today, sprinkled with many distinctive mutations.
Working on a prevailing theory that man migrated out of Africa, they also analyzed genetic variants in 846 people of non-African heritage, 176 people from sub-Saharan Africa, and a 50,000-year-old Neanderthal whose high-quality genome sequence the team published in 2013. The most powerful information the researchers used to determine whether a gene variant came from a Neanderthal was if the variant appeared in some non-Africans and the Neanderthal but not in the sub-Saharan Africans. Using this and other types of information, the team found that some areas of the modern non-African human genome were rich in Neanderthal DNA, which may have been helpful for human survival, while other areas were more like "deserts" with far less Neanderthal ancestry than average.
What the DNA Evidence Tells Us About Multiple Migrations Out of Africa
Comparing the full genomes of Neanderthal and Homo Sapiens concluded that most Europeans and Asians have between 1 to 4 percent Neanderthal DNA. Indigenous sub-Saharan Africans have no Neanderthal DNA because their ancestors did not migrate through Eurasia. It's possible that the ancestors of Neanderthals expanded out of Africa then, while our own ancestors stayed behind. The Neanderthal and modern human sequences differed by approximately 27.2 substitutions. The distinctive Neanderthal mutations discovered in their genome can be thought of as accumulating in a clock-like way, and by tallying them up, Pääbo and his colleagues estimate that Neanderthals and humans share a common ancestor that lived 800,000 years ago. It's likely Neanderthals, Denisovans, and modern humans are all descended from Homo heidelbergensis. Then between 300,000 and 400,000 years ago, an ancestral group of Homo heidelbergensis separated themselves shortly after they had left Africa. One group branched northwest into Europe and West Asia, which eventually evolved into Neanderthals. The other group ventured eastwards throughout Asia, eventually developing into Denisovans. Homo heidelbergensis evolved into Homo sapiens approximately 130,000 years ago. Between 70,000 to 60,000 years ago, modern humans began their journey out of Africa, expanding into Eurasia and encountering their ancient cousins.
While humans and Neanderthals crossed, they didn't crossbreed very often--at least according to this study. (ref: http://www.pnas.org/content/108/37/15129.abstract). However, researchers discovered that about 20 percent of the Neanderthal genome could be found in modern humans. Although the majority of genes inherited from Neanderthals apparently do not do anything remarkably different from their modern-human counterparts, some are beneficial. And although 20 percent might sound like a lot of mingling happened between Neanderthals and the ancestors of modern humans, it could have resulted from as few as 300 mating events.
On one level, it’s not surprising that modern humans were able to interbreed with their close cousins. The revelation that our ancient ancestors mated with one another could help explain one of the great mysteries in anthropology of why the Neanderthals apparently disappeared? It seems that after first venturing out of Africa, Neanderthals thrived in Europe for several hundred thousand years. But they mysteriously died out about 30,000 years ago, roughly around the same time that modern humans arrived in Europe. Some scientists have suggested modern humans out competed or outright killed the Neanderthals. But this new genetic evidence provides support for a more likely theory that the Neanderthal lineage disappeared because it was absorbed into the much larger human population.
What the DNA Evidence Tells Us About our Relationship to Neanderthal
The research suggests that humans and neanderthals were right on the edge of being different species, and their offspring may have been of low fertility - similar to what happens when donkeys and horses breed, but to a lesser degree. This conclusion is based on the lack of neanderthal genes on modern human X and Y chromosomes. Here is a quote:
"The team showed that the areas with reduced Neanderthal ancestry tend to cluster in two parts of our genomes: genes that are most active in the male germline (the testes) and genes on the X chromosome. This pattern has been linked in many animals to a phenomenon known as hybrid infertility, where the offspring of a male from one subspecies and a female from another have low or no fertility. 'This suggests that when ancient humans met and mixed with Neanderthals, the two species were at the edge of biological incompatibility,' said Reich, who is also a senior associate member of the Broad Institute and an investigator at the Howard Hughes Medical Institute. Present-day human populations, which can be separated from one another by as much as 100,000 years (such as West Africans and Europeans), are fully compatible with no evidence of increased male infertility. In contrast, ancient human and Neanderthal populations apparently faced interbreeding challenges after 500,000 years of evolutionary separation."
But there are also big stretches of the modern human genome that contain no Neanderthal DNA at all. They might provide a road map to positions in the genome that endow uniquely human traits. Where they are absent, natural selection may have decided that the modern versions were doing a better job and so eliminated the Neanderthal counterparts. Hence these “deserts” may help define the most important genetic differences between us and them. There are deserts on the X chromosome and in genes involved in the testes. Patterns like these in other species ranging from rabbits to fruit flies are regarded as signs that one species is about to split into two. Hybrid males descended from both branches tend to be infertile, like mules. That’s because males have only one X chromosome, and if it happens to be one that impairs their fertility, then they may not reproduce. Females have two X chromosomes, so even if one is impaired, if the other one is normal, it can rescue her ability to bear young.
Do Errors in Chromosome Pairing Define a New Species?
Biologists consider a species to be a group of animals that look similar and can only breed with each other successfully. DNA changes are one important factor in preventing closely related species from interbreeding. It's quite common to see some sort of large-scale change in the genome structure between species.
Chromosomes are the name scientists give to a strand of DNA in a cell. Humans have 46 chromosomes in most cells. These chromosomes come in pairs meaning that humans have 23 pairs of chromosomes. We have a pair of chromosome Ones, a pair of chromosome Twos, a pair of chromosome Threes and so on. We get one of each pair from mom and one from dad. So your mom gave you one chromosome One, one chromosome Two, and so on. Your dad gave you the other chromosome One, the other chromosome Two and so on.
The structure of the chromosome can change in many ways. Chromosomes can break apart, and they can fuse together. A part of a chromosome can flip around or a part can move to an entirely different chromosome. These sorts of things happen all the time. For example, some people have their chromosome 21 attached to another of their chromosomes. increasing the chance that they will have a child with Down syndrome. A DNA change like this might have helped separate humans from chimpanzees in our distant past.
If you look at the DNA sequence of a chimpanzee, it's 99% similar to us. Yet we look very different! If we compare the chromosomes, there's one big noticeable difference. In chimps, apes, and orangutans chromosome number Two seems to have split into two separate ones. The likeliest explanation is that our primate ancestor had two separate chromosomes. During human evolution those two chromosomes fused to create our chromosome Two. So what does that mean for evolution? Well, the fact that our chromosome structure is different is one major factor that would keep human and ape species from interbreeding. One important part of sexual reproduction is having an even number of chromosomes. Whether each chromosome has a partner chromosome is crucial. Whenever there are odd numbers of chromosomes or mismatched pieces, there will likely be mistakes during cell division. At the last step of meiosis, one chromosome from each pair moves into the new cells. An odd number of chromosomes means that one chromosome is left unpaired. It can go to either cell. The end result is that cells end up with the wrong number of chromosomes. And this leads to sterility or birth defects.
Conclusions About Human-Neanderthal Compatibility
Since biology defines a species as a group of organisms that can interbreed and produce fertile offspring. How would it be possible then for Homo Sapiens to produce fertile offspring with Homo neanderthalensis if we are not the same species? According to Ed Green, leader on the Neanderthal Genome Sequencing Consortium and Professor of Biomolecular Engineering at the University of California, Santa Cruz:
"By that strict definition, they are not separate species. According to that same definition brown bears and polar bears are not separate species either since hybrids of these two exist."
It is also noted that the genetic variations between Neanderthal and Humans is no greater than that of two chimpanzees. But chips and other animals also have greater genetic variation than humans. So would this imply two species of humans then if some of us have Neanderthal genes? Again, according the Profession Green:
"The concept of species was useful in the past for classifying life on our planet. Today, we know much more about how life forms reproduce and evolve and the concept of species has become less useful if at all. Creating any definition of species you want will inevitably lead to clear violations of the definition."
For now, it appears we must be content with the idea that genetically Neanderthal is evidently different than us by about 27 substitutions in the genome, but not so far removed as to fail the classical test of a different species.