By >Daniel Zadik , Leicester University 

The DNA of Albert Perry may change the story of human origins. Perry, an African-American, approached a DNA testing company to find out more about his ancestry. >The results would have come as quite a surprise (had he lived to see them), and have raised questions >for geneticists around the world. 

It turns out that Perry carried a very different type of Y chromosome, never seen before. Every male has a Y chromosome, which is a piece of DNA inherited by sons from their fathers. But, unlike most DNA, the Y chromosome is not shuffled as it is passed down, and changes only slowly through mutation. Tracking these mutations allows scientists to create a genetic tree of fathers and sons going back through time.

As a man may have several sons or none, some branches of the genetic tree die out each generation, while others become more common. Going back through time it is therefore inevitable that all modern Y chromosomes must descend from from one man at some point in the past. He has become known as “Y-chromosomal Adam”.

This Adam was not the first man, or the only man, from his time to contribute to modern human DNA. It is just that, by chance, his Y chromosome was the only one to survive until today.

What is surprising about Perry’s Y chromosome is that it did not descend from Y-chromosomal Adam’s. Or rather that the established “Adam” has lost his title to a new “Adam”, further back in time, where Perry’s branch split from the tree (see figure). While the former-Adam is estimated to have lived around 202,000 years ago, the revised one is thought to be about 338,000 years old.

To find where Perry’s Y chromosome may have come from, samples from around Africa were tested. Several more from Perry’s branch were found amongst the Mbo people of Cameroon.

So can this tell us anything about human origins? Central Africa contains Y chromosomes from both Perry’s branch and the former-Adam’s branch, while the rest of the world has only been shown to contain the former-Adam’s branch (with the exception of Perry himself). This suggests that our revised Adam may have lived in Central Africa.

The oldest-known “modern human” bones are from East Africa. But if Adam lived in Central Africa, does that mean that modern humans could have originated there? Again, it is hard to say. By looking further into the >genetics of modern people , the picture becomes even more complex. 

It so happens that, just like the Y-chromosome is passed down only from father to son, there is a piece of DNA which sits in a different part of the cell called mitochondria, that is passed down only from mother to her children. Tracing back this DNA in a similar way, leads us to a “ >Mitochondrial Eve ”, estimated to have lived about 190,000 years ago. Eve possibly lived in south-eastern Africa. But modern humans have DNA both from Adam and Eve. 

Despite these apparent contradictions, it is possible that modern humans descended from a single localised population, and that geographical differences in diversity today are due to spread and extinction in the intervening years. But it could also be that many of >the genetic and cultural ingredients that produced modern humans existed in different parts of Africa, drifting and spreading until they came together and, by a mixture of luck and natural selection, became the combination that would out-compete their relatives to spread to the rest of the world. 

One way or another, around 200,000 years ago, bones appear that are indistinguishable from today’s. But that is 140,000 years later than the estimated age of the new Adam, leading to the question: was he even “human”?

Answering this is tricky. There was no single moment when we became human, but rather a >gradual process . On an evolutionary timescale, Adam was very recent, and even if he was not “anatomically modern”, he could probably walk down a street today without raising too many eyebrows. 

Given the scarcity of Perry’s branch and the lack of diversity within it, it is also possible that the revised Adam could have been an ancestor of two sub-species (or even species). Could one have become modern humans, while another produced a cousin? What if, long after modern humans had become established and started to spread, they should meet and interbreed? Like all our other close relatives, these cousins eventually disappeared, but maybe they left traces, such as Perry’s Y chromosome, in the modern gene pool.

This may sound shocking, but it would not be unprecedented. When modern humans spread from Africa to Eurasia, they met another cousin, the Neanderthals. Fossils with features from both species have long caused debate, and recently genetic evidence has suggested that today’s non-Africans owe 1 to 4% of their ancestry to >such interbreeding , although no Y chromosomes have yet been identified. 

So, like many discoveries, Perry’s Y chromosome raises more questions than it answers. It will doubtless be fascinating to watch our understanding evolve as the genetics of more individuals, modern and ancient, from more locations are added to the picture.

Daniel Zadik does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations. 

This article was originally published at >The Conversation . Read the >original article . 

Published – July 22, 2013 06:37 pm IST

Neanderthals, the closest cousins of modern humans, lived in parts of Europe and Asia until their extinction some 30,000 years ago.

Genetic studies are revealing ever more about the links between modern humans and these long-gone relatives – most recently that a rush of interbreeding between our species occurred in a relatively short burst of time around 47,000 years ago. But one mystery still remains.

The Homo sapiens genome today contains a little bit of Neanderthal DNA. These genetic traces come from almost every part of the Neanderthal genome – except the Y sex chromosome, which is responsible for making males.

So what happened to the Neanderthal Y chromosome? It could have been lost by accident, or because of mating patterns or inferior function. However, the answer may lie in a century-old theory about the health of interspecies hybrids.

Neanderthal sex, genes and chromosomes

Neanderthals and modern humans went their separate ways somewhere between 550,000 and 765,000 years ago in Africa, when Neanderthals wandered off into Europe but our ancestors stayed put. They would not meet again until H. sapiens migrated into Europe and Asia between 40,000 and 50,000 years ago.

Scientists have recovered copies of the full male and female Neanderthal genomes, thanks to DNA from well-preserved bones and teeth of Neanderthal individuals in Europe and Asia. Unsurprisingly, the Neanderthal genome was very similar to ours, containing about 20,000 genes bundled into 23 chromosomes.

Like us, they had two copies of 22 of those chromosomes (one from each parent), and also a pair of sex chromosomes. Females had two X chromosomes, while males had one X and one Y.

Y chromosomes are hard to sequence because they contain a lot of repetitive “junk” DNA, so the Neanderthal Y genome has only been partially sequenced. However, the large chunk that has been sequenced contains versions of several of the same genes that are in the modern human Y chromosome.

In modern humans, a Y chromosome gene called SRY kickstarts the process of an XY embryo developing into a male. The SRY gene plays this role in all apes, so we assume it did for Neanderthals as well – even though we haven’t found the Neanderthal SRY gene itself.

Interspecies mating left us with Neanderthal genes

There are lots of little giveaways that mark a DNA sequence as coming from a Neanderthal or a H. sapiens. So we can look for bits of Neanderthal DNA sequence in the genomes of modern humans.

The genomes of all human lineages originating in Europe contain about 2% Neanderthal DNA sequences. Lineages from Asia and India contain even more, while lineages restricted to Africa have none. Some ancient Homo sapiens genomes contained even more – 6% or so – so it looks like the Neanderthal genes are gradually fading out.

Most of this Neanderthal DNA arrived in a 7,000-year period about 47,000 years ago, after modern humans came out of Africa into Europe, and before Neanderthals became extinct about 30,000 years ago. During this time there must have been many pairings between Neanderthals and humans.

At least half of the whole Neanderthal genome can be pieced together from fragments found in the genomes of different contemporary humans. We have our Neanderthal ancestors to thank for traits including red hair, arthritis and resistance to some diseases.

There is one glaring exception. No contemporary humans have been found to harbour any part of the Neanderthal Y chromosome.

What happened to the Neanderthal Y chromosome?

Was it just bad luck that the Neanderthal Y chromosome got lost? Was it not very good at its job of making males? Did Neanderthal women, but not men, indulge in interspecies mating? Or was there something toxic about the Neanderthal Y so it wouldn’t work with human genes?

A Y chromosome comes to the end of the line if its bearers have no sons, so it may simply have been lost over thousands of generations.

Or maybe the Neanderthal Y was never present in interspecies matings. Perhaps it was always modern human men who fell in love with (or traded, seized or raped) Neanderthal women? Sons born to these women would all have the H. sapiens form of the Y chromosome. However, it’s hard to reconcile this idea with the finding that there is no trace of Neanderthal mitochondrial DNA (which is limited to the female line) in modern humans.

Or perhaps the Neanderthal Y chromosome was just not as good at is job as its H. sapiens rival. Neanderthal populations were always small, so harmful mutations would have been more likely to accumulate.

We know that Y chromosomes with a particularly useful gene (for instance for more or better or faster sperm) rapidly replace other Y chromosomes in a population (called the hitchhiker effect).

We also know the Y chromosome is degrading overall in humans. It is even possible that SRY was lost from the Neanderthal Y, and that Neanderthals were in the disruptive process of evolving a new sex-determining gene, like some rodents have.

Was the Neanderthal Y chromosome toxic in hybrid boys?

Another possibility is that the Neanderthal Y chromosome won’t work with genes on other chromosomes from modern humans.

The missing Neanderthal Y may then be explained by “Haldane’s rule”. In the 1920s, British biologist J.B.S. Haldane noted that, in hybrids between species, if one sex is infertile, rare or unhealthy, it is always the sex with unlike sex chromosomes.

In mammals and other animals where females have XX chromosomes and males have XY, it is disproportionately male hybrids that are unfit or infertile. In birds, butterflies and other animals where males have ZZ chromosomes and females have ZW, it is the females.

Many crosses between different species of mice show this pattern, as do feline crosses. For example, in lion–tiger crosses (ligers and tigons), females are fertile but males are sterile.

We still lack a good explanation of Haldane’s rule. It is one of the enduring mysteries of classic genetics.

But it seems reasonable that the Y chromosome from one species has evolved to work with genes from the other chromosomes of its own species, and might not work with genes from a related species that contain even small changes.

We know that genes on the Y evolve much faster than genes on other chromosomes, and several have functions in making sperm, which may explain the infertility of male hybrids.

So this might explain why the Neanderthal Y got lost. It also raises the possibility that it was the fault of the Y chromosome, in imposing a reproductive barrier, that Neanderthals and humans became separate species in the first place.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Neanderthals created stone tools held together by a multi-component adhesive, a team of scientists has discovered. The astonishingly well-preserved tools, more than 40,000 years old, showcase a technical solution broadly similar to examples of tools made by early modern humans in Africa. Its findings, which are the earliest evidence of a complex adhesive in Europe, suggest these predecessors to modern humans had a higher level of cognition and cultural development than previously thought. The stone tools from Le Moustier — used by Neanderthals during the Middle Palaeolithic period of the Mousterian between 120,000 and 40,000 years ago — are kept in the collection of Berlin’s Museum of Prehistory and Early History and had not previously been examined in detail. The tools were rediscovered during an internal review of the collection and their scientific value was recognised. The researchers discovered traces of a mixture of ochre (over 50%) and bitumen on several stone tools. Using liquid bitumen with 55% ochre, researchers were able to produce a mixture that was sticky enough for a stone tool to remain together but without adhering to hands.