Daily Archives: 8 September 2011

Neanderthals are partly us

When modern Homo sapiens left their evolutionary home in Africa, they (we?) encountered and eventually displaced the older species Homo neanderthalensis.

But it wasn’t all fightin’ and killin’ – recent genetic sequencing has shown that to some extent our ancestors interbred with Neanderthals, and that this probably helped their survival by boosting their immune systems.

When the Neanderthal genome sequence was published in 2010, parts of it were shown to be common to most modern humans. In fact, people from all over the world – yes, even Australia – shared 1-4% of their genes with Neanderthals; everywhere that is, except Africa (Ref 1).

This suggests that when modern humans left Africa – commonly believed to be about 70,000 years ago – they met and mated with Neanderthals in the Middle East. Whereas those who stayed in Africa didn’t meet Neanderthals and so didn’t pick up their genes (at least at first).

Map of the global distribution of the HLA-A gene, believed to have originated in Neanderthals, showing how it's far more common in Europe, Australia and especially East Asia, compared to Africa (click to embiggen)
Map of the global distribution of the HLA-A gene, believed to have originated in Neanderthals. (Image from Science/AAAS)

Now, new research has shown that some of the genes we share are important for our immune system (Ref 2).

These genes are part of the human leukocyte antigen system, or HLA. The HLA genes, or rather the proteins created from their code, are used by our immune systems to distinguish our cells from invaders. Picking up these new genes from locals who had already adapted to their environments would have helped our ancestors fight the many new diseases they encountered as they moved around.

But the HLA genes didn’t come from just one source: some are also common to the Denisovans, a newly discovered species whose mitochondrial DNA – sequenced from toe and finger bones found in Siberia – is distinct from both Neanderthals and modern humans (Ref 3).

What’s even more startling is that, even though the Neanderthal and Denisovan genes are most common in Eurasians, they are actually found to a small degree in Africans too, implying that they were brought back in at a later point.

This prehistoric promiscuity and the propensity to spread genes around definitely seems to have made a contribution to human success, and the outlasting of our hominin predecessors.

References:

  1. Green RE, Krause J, Briggs AW, Maricic T, Stenzel U, Kircher M, Patterson N, Li H, Zhai W, Fritz MH, Hansen NF, Durand EY, Malaspinas A-S, Jensen JD, Marques-Bonet T, Alkan C, Prüfer K, Meyer M, Burbano HA, Good JM, Schultz R, Aximu-Petri A, Butthof A, Höber B, Höffner B, Siegemund M, Weihmann A, Nusbaum C, Lander ES, Russ C, Novod N, Affourtit J, Egholm M, Verna C, Rudan P, Brajkovic D, Kucan Ž, Gušic I, Doronichev VB, Golovanova LV, Lalueza-Fox C, de la Rasilla M, Fortea J, Rosas A, Schmitz RW, Johnson PLF, Eichler EE, Falush D, Birney E, Mullikin JC, Slatkin M, Nielsen R, Kelso J, Lachmann M, Reich D & Pääbo S 2010, “A draft sequence of the Neandertal genome”, Science, vol. 328, no. 5979, pp. 710-722, doi:10.1126/science.1188021
  2. Abi-Rached L, Jobin MJ, Kulkarni S, McWhinnie A, Dalva K, Gragert L, Babrzadeh F, Gharizadeh B, Luo M, Plummer FA, Kimani J, Carrington M, Middleton D, Rajalingam R, Beksac M, Marsh SGE, Maiers M, Guethlein LA, Tavoularis S, Little A-M, Green RE, Norman PJ & Parham P 2011, “The shaping of modern human immune systems by multiregional admixture with archaic humans”, Science, Published online 25 August 2011, doi:10.1126/science.1209202
  3. Krause J, Fu O, Good JM, Viola B, Shunkov MV, Derevianko AP & Pääbo S 2010, “The complete mitochondrial DNA genome of an unknown hominin from southern Siberia”, Nature 464, pp. 894-897, doi:10.1038/nature08976

Higgs update

One of the most publicised goals for the Large Hadron Collider (LHC) in Switzerland was to search for the Higgs boson, the hypothetical particle that could explain how other subatomic particles get their mass.

Well, the LHC has been running for nearly 18 months now, so what has it found? Any sign of the Higgs?

The LHC’s two general purpose experiments, ATLAS and CMS, announced their preliminary findings at August’s Lepton-Photon conference, held in Mumbai, India. And so far it’s not looking good for the Higgs boson.

(There are actually six experiments running at the LHC. As the name suggests, the Large Hadron Collider is a very large particle accelerator that collides hadrons, i.e. protons, at very high energy. Examining the outcome of these collisions are a range of detectors, run by six different groups; these make up the LHC’s six experiments.)

Plots of results from the Higgs search by the ATLAS experiment, showing how most masses under 466 Gev are ruled out (click to embiggen)
Plot of ATLAS experiment's Higgs boson search results to date. The wobbly black dashed line shows the prediction from simulations, and the green and yellow bands are the uncertainty in the predictions (at one and two standard deviations respectively). The solid black line is the actual result from the collected data. The Higgs boson is ruled out with 95% confidence wherever the solid black line dips below the horizontal line at 1. (Image ATLAS)

Things were looking promising a couple of months ago, when there were a couple of anomalous signals that could have been the Higgs, but twice as much data has been collected since and the signals have faded.

At the moment, there’s still a slight chance the Higgs exists with a mass between 115 and 145 GeV, or maybe 232-256 GeV, or possibly 282-296 GeV, or potentially even above 466 GeV. All other mass ranges have been ruled out with 95% confidence (GeV, or giga-electron volt, is the unit used to measure the mass of subatomic particles; protons have a mass of 0.938 Gev).

There’s still more data to collect though, so it’s possible there’ll be a definite result either way by the end of the year.

But it’s not necessarily a bad thing if the Higgs isn’t discovered: it simply means that some other, unexpected mechanism must explain the origin of mass. Which is kind of good, because it means new physics, and the potential for new discoveries.

What those might be is unknown – and in fact so far the LHC has ruled out a few popular theoretical possibilities. But that just means the real truth is likely to be something unexpected, and that’s where the really interesting discoveries come from.

All of which makes it an exciting time to be a particle physicist – not that it ever isn’t.

Read more about the results from the ATLAS experiment