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The origin of human immunodeficien-cy virus type 1 (HIV-1), the retrovirusthat is the main cause of AIDS, has

been a puzzle ever since it was discovered byBarré-Sinoussi and her colleagues1 in 1983.On page 436 of this issue2, Gao et al. providethe most persuasive evidence yet that HIV-1came to humans from the chimpanzee, Pantroglodytes, which harbours a related simianimmunodeficiency virus, SIVcpz. Moreover,by genetic typing of mitochondrial DNA, it isclear that the three strains of SIVcpz mostclosely related to HIV-1 come from a singlesubspecies, Pan troglodytes troglodytes,which lives in the same part of central Africawhere AIDS is thought to have arisen.

There are three principal groups of HIV-1: the main (M) group comprises the majori-ty of subtypes that have spread across theworld; an outlier (O) group is found inCameroon, Gabon and Equatorial Guinea;and a new group (N) was last year identifiedin two people in Cameroon3. HIV geneticsequences from a Congolese serum sample,originally taken in 1959, showed that thediversification of all the M subtypes hasoccurred in very recent times, say the past 50years4. Gao and colleagues’ evolutionaryanalysis of the M, N and O groups in com-parison to SIVcpz isolates indicates thatthese HIV-1 groups represent three separatetransfers from chimpanzees to humans(such inter-species transmissions of infec-tious disease are known as zoonoses).

It has been shown5,6 previously that sev-eral strains of HIV-2 in West Africa were in-dependently derived from SIVsm, the virusendemic in the sooty mangabey monkeysof that region. The other human retroviralpathogen, human T-lymphotropic virustype 1 (HTLV-1), has also originated morethan once from related simian viruses,including STLV-1 of chimpanzees7,8. And anew paper9 reporting a parallel zoonosis tothat of Gao et al.2 has demonstrated thathuman pygmies in Gabon are infected with astrain of HTLV-1 that is virtually indistin-guishable from STLV-1 of mandrills living inthe same forest. Finally, a survey of peoplewho handle primates in captivity recordedseveral instances of foamy retrovirus trans-mission and one transfer of SIVmac10, thestrain that causes AIDS in macaques.

So cross-species transmission of primateretroviruses to humans through huntingor husbandry probably occurs relativelyfrequently, although — thankfully — thesubsequent epidemic spread in the humanpopulation is a much rarer event. Suchzoonoses have heightened the concern thatretroviruses might also cross to humans frommore distant species, such as pigs, if theirtissues were used for xenotransplantation11.

Gao et al.2 also show that a chimpanzeecalled Noah, from which a highly divergentstrain, SIVcpzANT, was isolated, actuallybelongs to the different subspecies Pantroglodytes schweinfurthii (the names and dis-tributions of all four subspecies are shown inthe map on page 438). Gao et al. argue thatSIVcpz is an ancient chimpanzee infectionthat has co-evolved with its host during sub-speciation. They previously followed the samelogic for the diversification of SIVagm amongthe various subspecies of African green mon-key12. But these monkeys, like the chimpanzeesubspecies, have largely non-overlapping

habitats, so it is also conceivable that SIVs weremore recently introduced and owe their diver-sity to geography rather than co-evolution.

Indeed, the ability of SIV to jump hostspecies, together with the demonstration byGao et al. that one strain of SIVcpz is (likeHIV-1E in Thailand) derived from geneticrecombination between two distinct parentstrains, suggests that the co-evolutionhypothesis needs to be viewed with a littlecaution. To bolster Gao and colleagues’conclusion that chimpanzees are a naturalreservoir population for the precursor ofHIV-1, serological surveys for the preval-ence of SIVcpz infection are required inwild populations in Africa.

The ramifications of the new findings2

extend well beyond the evolution of HIV-1,and zoonotic transmission of disease, tochimpanzee conservation and welfare. Likethe other great apes, chimpanzees are in trou-ble. In most of the 21 African countries wherethey live in the wild, their numbers are plum-meting — partly because of habitat loss, andpartly because ape-hunting has become bigbusiness (Fig. 1). Hunters are paid by timbercompanies to provision logging camps, whilethe kills of freelance hunters are traded as faras the cities. Ape meat can fetch premiumprices in middle-class restaurants.

This ‘ape bushmeat crisis’ cannot lastlong. Numbers are hard to come by, but someestimates of the annual chimpanzee kill arein the thousands, which is an unsustainablyhigh figure when as long as a decade ago theworld population was estimated at around200,000. In the face of such an onslaught,populations of the large, slowly reproducingapes will be swiftly eradicated13.

Hunters dismember chimpanzees withprimitive butchery, and so expose them-selves to the risk of zoonotically transmitteddisease. Conservationists, therefore, aredebating the merits of a campaign thatwould publicize the disease-transmittingdangers of treating apes as food. Some areconcerned that this could backfire, and resultin intensified slaughter of the primates,whereas others think the situation couldhardly get worse. Later this month, a meetinghosted by the American Zoological Associa-tion aims to turn discussion into action. Inthe long term, apes will survive in the wildonly where they are not eaten, a prospect thatdepends on the acceptance of new values bywhich apes are treated as too human-like tokill or eat. Many believe that new ethics arealso needed in the scientific world. To someAIDS researchers, the use of chimpanzees isconstrained merely by their high cost andscarcity14. By 1996, 198 chimpanzees hadbeen experimentally infected with HIV insix of the major research institutions inthe United States15, although HIV seldomcauses disease in these animals.

Worldwide, there are now about 35 mil-lion carriers of HIV-1, and with the increased

NATURE | VOL 397 | 4 FEBRUARY 1999 | www.nature.com 385

news and views

From Pan topandemicRobin A. Weiss and Richard W. Wrangham

Further evidence that HIV-1 originally came from chimpanzees bears ona variety of issues — the evolution of AIDS viruses, diseasetransmission from animals to humans, and chimpanzee conservationand welfare.

Figure 1 Chimpanzee orphan, chained in a junkcar lot in Cameroon. The rest of its family hadbeen shot and butchered for the bushmeat trade.

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evidence that the virus is linked to SIVcpzthere may be calls for more use of chim-panzees in AIDS research. But according to agrowing constituency, evidence of ape posses-sion of human-like cognition and emotionsshould reduce our willingness to infect them,and condemn them to solitary confinementfor life (which lasts for 60 years or more)16.

The approach of Gao et al. may promptfresh thinking. Fieldwork with free-livingapes provided information for the evolu-tionary history of chimpanzee subspeciesthat was critical to their results. Further field-work linking demographic data to biomed-ical monitoring could be even more produc-tive. The four chimpanzee subspecies havedeep evolutionary roots, and may yieldfurther types of SIV beyond the two alreadyidentified. These are best studied in the placewhere the host–virus systems evolved.Chimpanzees in captivity are mostly takenfrom the wild before they become sexuallyactive, and so rarely harbour SIV. Infectioncan be expected at higher frequencies in wildadults17, and such individuals would offerresearch opportunities that simply do notpertain in captivity, where experimenterscannot know how virus strains match togenetic lineages.

Two other ways forward are these. Fieldresearchers intermittently find fresh corpsesof individual apes, often with well-docu-mented life-histories, which offer untappedpossibilities for virological analysis. AndDNA studies of faeces might allow non-invasive monitoring of virus infection.

Biomedical researchers have the prospectof collaborating with fieldworkers in a syn-thesis that would benefit conservation. Thelink between HIV-1 and SIVcpz may open adoor for research that helps both humansand apes.Robin A. Weiss is in the Windeyer Institute,University College London, 46 Cleveland Street,London W1P 6DB, UK.e-mail: robinw@icr.ac.ukRichard W. Wrangham is in the Department ofAnthropology, Harvard University, Cambridge,Massachusetts 02138, USA.e-mail: wrangham@fas.harvard.edu 1. Barré-Sinoussi, F. et al. Science 220, 868–871 (1983).

2. Gao, F. et al. Nature 397, 436–441 (1999).

3. Simon, F. et al. Nature Med. 4, 1032–1037 (1998).

4. Zhu, T. et al. Nature 391, 594–597 (1998).

5. Gao, F. et al. Nature 358, 495–499 (1992).

6. Chen, Z. et al. J. Virol. 71, 3953–3960 (1997).

7. Koralnik, I. J. et al. J. Virol. 68, 2693–2707 (1994).

8. Voevodin, A. et al. Virology 238, 212–220 (1997).

9. Mahieux, R. et al. J. Virol. 72, 10316–10322 (1998).

10.Heneine, W. et al. Nature Med. 4, 403–407 (1998).

11.Weiss, R. A. Nature 391, 327–328 (1998).

12.Sharp, P. M., Robertson, D. L. & Hahn, B. H. Phil. Trans. R. Soc.

B 349, 41–47 (1995).

13.Wilkie, D. S. & Carpenter, J. Biodivers. Conserv. (in the press).

14.Letvin, N. L. Science 280, 1875–1880 (1997).

15.National Research Council Chimpanzees in Research: Stategies

for their Ethical Care, Management, and Use (NRC, Washington,

DC, 1996).

16.Singer, P. & Cavalieri, P. (eds) The Great Ape Project (St

Martin’s, New York, 1994).

17. Jolly, C. J. et al. J. Med. Primatol. 25, 78–83 (1996).

news and views

386 NATURE | VOL 397 | 4 FEBRUARY 1999 | www.nature.com

Condensed-matter physics

Return of the itinerant electronDavid Ceperley

On page 412 of this issue, Young et al.1

demonstrate magnetic polarizationin a dilute, three-dimensional elec-

tron gas at a temperature of 600 K. Years oftheoretical debate connect this intriguingresult to a long-sought mechanism for ferro-magnetism in metals first conjectured byBloch2, who suspected that the ‘electron gas’is susceptible to magnetic ordering at lowdensity.

What is an electron gas and why is it inter-esting? The idealized homogeneous gas (alsoknown as jellium or the one-componentplasma) has a long history going back to thediscovery of the electron. It was used by theearly quantum pioneers to model the valenceelectrons in a solid; they replaced the ions by arigid uniform background of positive charge.This is arguably the most important model incondensed-matter physics because it is rou-tinely used as a reference state in most realis-tic calculations of electronic structure. Thevalence electrons are the outer electrons ofthe atom, which determine how it interactswith its environment, so understanding theproperties of the electron gas is significant. Inmost metals, the density is high enough thatthe electrons move almost independentlyand form a ‘normal Fermi liquid’ of mobileelectrons (Fig. 1) with equal numbers of elec-trons with up and down spin. But at low den-

sity the potential energy dominates the kinet-ic energy, and the electron ‘gas’ freezes intothe ‘Wigner crystal’3, with the electrons local-ized on lattice sites. This dependence on den-sity is the exact opposite found in systemswith cores, such as atoms, where it is high notlow density that favours a lattice structure.

In 1929, Bloch noticed that within the‘Hartree–Fock’ approximation (that is,assuming the electrons are in independentstates), the spins of the electrons sponta-neously align, and he suggested this as anexplanation of ferromagnetism. This spinpolarization should happen at a density justlower than that found in some alkali metals,such as potassium, rubidium or caesium.But at these densities one cannot ignore thefact that electrons move in a correlated way.In the Stoner model4 introduced in the 1930s,the electron–electron Coulomb interactionis replaced by a constant repulsive energybetween opposite spin electrons, to accountfor this correlation. The spins only partiallypolarize and they do so at a lower density.Many theorists have concluded from this andother calculations that the electron gas wouldnever be ferromagnetic5. Calculations at anintermediate density are difficult becausevery small energy differences are importantand any approximation has to treat the vari-ous ‘phases’ of the gas with equal accuracy.

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Figure 1 Phase diagram of the electron gas. The two colours divide the classical (blue) from thequantum (yellow) regimes. The phase transition boundaries are estimates from ref. 6. The dot is thetransition temperature measured by Young et al.1.