CINXE.COM

<!DOCTYPE html><html lang="en-US"><head><title>Just a moment...</title><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=Edge"><meta name="robots" content="noindex,nofollow"><meta name="viewport" content="width=device-width,initial-scale=1"><style>*{box-sizing:border-box;margin:0;padding:0}html{line-height:1.15;-webkit-text-size-adjust:100%;color:#313131;font-family:system-ui,-apple-system,BlinkMacSystemFont,Segoe UI,Roboto,Helvetica Neue,Arial,Noto Sans,sans-serif,Apple Color Emoji,Segoe UI Emoji,Segoe UI Symbol,Noto Color Emoji}body{display:flex;flex-direction:column;height:100vh;min-height:100vh}.main-content{margin:8rem auto;max-width:60rem;padding-left:1.5rem}@media (width <= 720px){.main-content{margin-top:4rem}}.h2{font-size:1.5rem;font-weight:500;line-height:2.25rem}@media (width <= 720px){.h2{font-size:1.25rem;line-height:1.5rem}}#challenge-error-text{background-image:url(data:image/svg+xml;base64,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);background-repeat:no-repeat;background-size:contain;padding-left:34px}@media (prefers-color-scheme:dark){body{background-color:#222;color:#d9d9d9}}</style><meta http-equiv="refresh" content="390"></head><body class="no-js"><div class="main-wrapper" role="main"><div class="main-content"><noscript><div class="h2"><span id="challenge-error-text">Enable JavaScript and cookies to continue</span></div></noscript></div></div><script>(function(){window._cf_chl_opt={cvId: '3',cZone: "www.science.org",cType: 'managed',cRay: '8e90f4e2fcb589b2',cH: '85wqceXdvqXSKiKt3.kW3RAEYJK7qQ.1DXpwi4Fy_ho-1732698327-1.2.1.1-zS1vpvbHLCeJWQaqDoEQ8geEzjbsp0I3UHeQ3MDHSirdC9k7sgUTZS.QnCcR0YOd',cUPMDTk: "\/doi\/10.1126\/science.177.4047.393?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub 0pubmed&__cf_chl_tk=OK.ih1fimFaILxZIbqv.k53oSM1mAtaJok0yg.NeRlA-1732698327-1.0.1.1-hjGzsRbFDEIKu0wurEQFF7.tXqXtdpfx2mlnwdU0qBE",cFPWv: 'b',cITimeS: '1732698327',cTTimeMs: '1000',cMTimeMs: '390000',cTplC: 0,cTplV: 5,cTplB: 'cf',cK: "",fa: "\/doi\/10.1126\/science.177.4047.393?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub 0pubmed&__cf_chl_f_tk=OK.ih1fimFaILxZIbqv.k53oSM1mAtaJok0yg.NeRlA-1732698327-1.0.1.1-hjGzsRbFDEIKu0wurEQFF7.tXqXtdpfx2mlnwdU0qBE",md: "4EYB4h9aQyx1j8HTuCMjU6mO283DFq1CMu.eLNDTL4g-1732698327-1.2.1.1-H_QiKF13ASy1i8wVGWFNKk8ezxsP15SQLx4jqkMcyHIRkhvpyInbvm4uw4CP6C3OmEW7O6ZBGZRDm0S9ASUhtgNnoA7wVSco14XjvDqNS38C0Zx3FxHUdhECRcnt_9seE.Z9WLkGBXsT3I4ysUUHJBZMyTopqWwLzyM6Bz9Ztcj.PuoeJmVk0akXtBJYPIAgBvZ8vOf2t_cpErOgG51pUFPzqoKVTak8hG5PvwZUx9n6C8B7zpcbtTLeeg2nmR.dXOd_zGscRG51tsB8nSQjgP0UZymnJ5R0fqKswnVjWQOSoIZHiBXweL3Os7nRbsHjJ1cLzRve7pAlrhkqFiSYuOS1rIHMdIn69mWX997uVvfZtAPhKQQYXLyd13t2PlLFn0HhgwUvbSibYEleZ4B4gzFtbbplgBqMi4lcrqn92XIPnZv81ALEo_f2Hdud22vDpt1CRs9fm4Cc2bQPqqQYVKB4IwSZKw5Etfdqsbx1tjFDuoAcIgS9AafYQR1jwMqiNm9oBj6hiwej.yULdmv_mFxVA0UmvHaW8zt_UiA0lpzYkCta9jPa8QaiLatdkX1YN8ZIcOnoP3ah2P8XaN_u4zTHkVkxAVOOtzfa5hEDqlgDlpOr7zYCfKmLviz8mhw1Eeq.QTy.KkDZG2YfO9F.vZf6qZe9JSAuT60ZqAhhLDw2teo_LOZQmqXLeZN_csskKZ5aT6pRp4Ea4KHvfHzfHPuOgFMzESeLnWwiypt7SOy7.j.JSKy1onZxabnpC95Vvu6piz09PET6lnjnAwEtfj6QmRuwsdXULOxLx.wCFqe7zWa1zOk6QCJNqnGuX84_TtSTsUxHtPU.GhkEu1Mk_Z69Dj2tcY89.dSafw6Q7ISaxU9wGnsPEUGXEX1sWG7mP6tFrbKtx.UDWqUwbwYDTALcRBKHUaapc9tDByXECZBSY3HxmJBgGljy4IrE8UAhFr5uAzgH2QVbz5oqrX1AcomGB5uU_k7d4SLMYr4ce8IvEx.p63n410ihFrL4LrlgKWFpRjvpCF4mDGsd6arMyHGydPGik8ZhyITiC1_5H7rJ2repLra.sfg_OvuMf2C4JJfcE4wTjTg03xxOsnC5pLiK3ol7MkaISU8HmgVc5Va1gaia4eb.j9CYWDOFfNCDGxcOVOIq86cQHCGy9ptIxb1sbkk_GCSn_j3AP.Bd1vmUMAyJG5sx_hxBnaEC6vEok0qmeqGZcqI8lMbtSjojevI.r9zxsYOmdX7z36qrt1hyo05_kwiNJPJF0ql3CUD_TRVpCFKtwI7DeGm.OXl9zNd5gmMktaDayU76uLlG7ThlWjo4XYSORfkgUct.acVBH63X7p_mcPcM31TfDs93qi7aRwuj6sxOqNXczze6fEufhojaeoiUow0PW4WJgeNJhtmWRM8TRXx6_eGnxITCCBSPWywr1x5rdhQbiCxGi2AvJKiJMZTA_MTo4_TlMtPfgqvnfmfLzVy.CYFC3KSvm2CJjJBKEySyroOR990EYv2NLS2x.rJScGSsJqPZHGbvSOX.e7UELH9u.BWUq8q.v0NtHoBH2mUdw356drGWTIsJkxU9zs55b7YDIJIRFvjcY0HUmqF_GvbheRdUcPSrS_hYmdiZMDmGwjQHZiv.SiVapLcWDQH8ViVSZZbuyEvINaGnVYsGVIhN9lrDA6NwyaOCLWsQx5YeVbrj7iwsOsUzykIVLDghzorPskxukXjG865aO4XkHHMAvQOAmHtN9mMaqBhspj8AEy_Co3Xp2Pm3YkLe8KJ98eSwX._S_CD8quAXf_0KAxEKY714DXbybgrqcf02A.NGuvgl5F.Z3x3_IFkgzTHWRTsNFX_Qi7bMOlD7owh6Yz4NhuI71oiznwhyRRZNBcLnt_.GlM2xQy6Euyg4YYrAmXTQmbgmG2pjM5V.iSt3XliT01a3yOVIDmwL.DwBf9uAh2AT8zuKYXyMTadloCbnDNDgjXchdQ5_5n_VNt6tAwVPk2LKTy49P0XVbncyHQX2AWoYH3GRQVNI7_jUX5jUNLpuyOd4uiRMUoyAPuB.QfCA.gAZ7xpZ9eHfQZfzxALXWFdckz7iE7aajVY4THQneoQpdjDL75t7EqpRp7uHVN_O6N3FhUMf9iMTFoOynb.iK6pCBE7Wm1qHLEFSPXgNXjg8IE3X4W04t44Lyru85QKHXvRW58nfZhlI5203bswUgl.aCooGB6Bp1O5RBZwJc0ZGsyxbCJewJB3kAnjsoUrjtwj1lJC4wSpF5zhZFUt85HY1CmK2ewUCrWSb1Pgm8u9RA00uVfsuep5IGk2fkzn4lKGugSEg9QTonBe2jw6ZoCLPIIDE5k3NyPrg_Uad0j.lG_d.drhInVw636Mi7xheOgsQPWs4gtjOaEGvVqBbd1VymAHfT5F88NzVQxSUT1ypDziCqnW3T2taK2vjcOO7nA2vHx_pMEt_2rE6igh_3S7nZqUvW015GH07KZiIUk0uGE4_CRymGRsE.3S4ILyKjOodzFPSsaAhPvOqnnCzG6KhByQXVEYOTIgbSHH9esNFemamThpB9aGeuxGRNzhZWhRi9nEDwrMCEKGP7ZCZv.Qvusef8uN5.f7xjt.hzRgO24GhoUfHJ0vbOxf9GIgBwokjnor4BFIf2LKYw89B3rgtWwskNzi52iumdputvwLNb6KOZzNmDIPf.YEjtp8YjEZvT_7HvYuDRuLxscmbwjmbuwKhk3jSKvr7ZtbKB3yHwIZri0WXBr9gAkNH872kunjit6svkwxZeZiMB_Q943kjjE782ub7BX37BzWLnwfsdoCoHye3QuOb0g2t99VAxgcqQHxjvaScl0tKGnrtwYiXTLK0pC00meM0NXXT7h5HUJ4FPQlhWW8nX4IyaPmvMOyS4qJbojkm2PoM6SkJTAHfq.wvTq4C1OKCKFb3u4wJg2fYvnXfq2RK2xj5OxQec9AeSxLvmFg0X4BnoMVy1sHrFwS0.eD2mXWUoDiCLPkM3ZMhjgqFiSksyD6SrXXEiIPqgii1qQTK0R_IGQAkReyabsg35HGe24ZpjHV9yYm5upVuKoqCu3_nZIpmfYN3gjY0xIgBkBds9ETHO9zvzIIXYcBpGkOulF7LCX6FaL6Kw6GoLWCDcX_wITH5rChKYJB6vKMw5BJ29v7AmJodL7Zj95m9R6My7SOohYl7DTBI1N0laZ6.",mdrd: "n.sx_q4RRmc20_y6fU.V_JWcHH6RfuX.ton1_rSICNk-1732698327-1.2.1.1-hy6umWcz7Beh6xam2xbQAP2WU2PxfLLcuvv.IqLIoA9DKC6PLnN.i9RE7ox8gXHRa04T0Ygeh1z5CI66ypDsV7xCu3zxzX4SKJLy21bckm98UVRoZcS3TnsNqAB_dfRaekr0_r0laJUmgUy_7Tb6bx8E7G6O2veLagFaC9oQKTAn76RgosnGgL1DXxOiMjPxSBl__5cNfFeBvUCvFSGuzZRZWvwVWKrGty_tuF9ASt3oTTlqemxmQ3XUzAVtaB1ZyusKxFfbbpgrquBA.XuSYlDPCw1pG5.qfMmPhRV7a2B9ENjftIWFVljL2tRjU47pWR9goZGRCVDLhOqJ4Vo9IhIh.j3eC6F5NR5Gxr5uWXwUQ3hDB.VG_fM7qjvBeGG29k39yAGSxpTtMeiV7ivoNg1kCHIBSMw4EmQQUTMbGR8IGTfp10TFxbpGCV84opH59DJBmVjPnFzWI8AXVFCdupKgUX8nu0dodh.uH6YP78DHfXXOjmfQuwRXTnPoADeIuRYT95fHcIxctMXT8zDmgQuzVZdRY3dlLPqE5VgvuHxyOVFqkNgutl3dvCwpT02518gyd1bLmHK.ZE8Nqg7pcR5WwFIjyQw0EofXgnskvrpBgVSA1o1dwFRkLAXYux5Jfaxllh2Y8404zEnfac5lHYsLAHDZSMxMLYWBpdw4w91BtPhq0lzLQ4IxBLp.nyEZzbnAHUl2UMOKo1mEuwaDfJWY_OLjNJAnxyUWlA.X3r4z3jtx7eNCLUtb0fSUgE0LB7XAFVcmpaZP9P6a_LvfLn09hXh5GaR03VTNgB02wkf21wvAbmoJOaA.d7Q5TXrYIUXrT7B9ef54RLMGZ5YvUcS_PrfQ2k4DvwSoA8.lCg4cHs.hy.zoPVh702gwf1oMPzn_RnZmsuFRE0mHa4piGjpjz4ndhUxEN.pmMFmk.JUbINI6jBv3Mb0dGIArzvJeWMcJ.cbA_.FuSUrVfViBXamZq.F90bnTgpCUYhtNxFf875c5WXl250yQHDytyif1hkHm9JkGBTXYk0u73jCTK10pWni3.N3dSi6XC81PF7067kfTq6q3bBP8sF1ixYI4EiS7eSRY.FAXs.Q8bu9y.nTQe2gr4bmlhloXAR0XMAFwLyDaei1yIlrNlRVEAtOmGUiZq4GbKusREcZoTlBUMF1JQ4FkQYQmwIz4uq785Mfw2DHAh4Ka6gLeTfTeVqy2ZY5j1M0yIQ7hqy0Iwx0UG6_mDYph49cvpCmTxHidV_9h.qJ.jUJna8SlYF_nAa58YXt4Yu2xQ5xI0L_NcJIFyQdVIWnyHEWvTj.X12RvWkRkg.snuZKFC6hxHKkLjhDaRiJAufj_Vj63nFw0SwajS1jkEjD8XBUjsoqwrGyJhWLUTKfaXAcHLki9QHKcqxeOTLhNY_hNQLng.82Eo_DGPvT_XcWdNCUCQTsoWIF0kM2xtl8FSUTBjHeF1uhSwNSzAHRZpTRdj1F.KrII4M9uE6zE1ste8pBwURHXWQwPn4nZAUn7F6dH9Mj0jTkdvQeV60HlCLrtumIs.wTz4AWinZNtrmI_KwXVkjRKEuKhPcpVHVYvZ3Fo0L1nw5UR8Si2OEB_KrISRcnLkiAUVD772NDd8lMXBNnUg1638zjrGZdea3T4hqCEmRwYk3G7HWs2iGIPTOP.OmZLWJf2I0xmnkUsuBc2lduP2c4YVwi3bvajSgRFqPutAbmn91cCoZrd90kt8DGnf4kRhyz7Td5IJsHvSGoyTqNG._V8qCM6X4Yeq3xcDKvceuMs3z5Rqf6BrazT8JhodRcrYSTY4J.aeSHG9kra3dij59Xj2W0RLhjV2TqpTAR7nDBOBz5WitCKac._mu2CwWrKQBly3gAYG_ljXteXvBo5A06f36FcUBuIV_BJdjonYLXxmDS89p9uLHAgcLyDuk0AsHfngLC2S_C14KN1dgO0tPBpd.ltMOc"};var cpo = document.createElement('script');cpo.src = '/cdn-cgi/challenge-platform/h/b/orchestrate/chl_page/v1?ray=8e90f4e2fcb589b2';window._cf_chl_opt.cOgUHash = location.hash === '' && location.href.indexOf('#') !== -1 ? '#' : location.hash;window._cf_chl_opt.cOgUQuery = location.search === '' && location.href.slice(0, location.href.length - window._cf_chl_opt.cOgUHash.length).indexOf('?') !== -1 ? '?' : location.search;if (window.history && window.history.replaceState) {var ogU = location.pathname + window._cf_chl_opt.cOgUQuery + window._cf_chl_opt.cOgUHash;history.replaceState(null, null, "\/doi\/10.1126\/science.177.4047.393?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub 0pubmed&__cf_chl_rt_tk=OK.ih1fimFaILxZIbqv.k53oSM1mAtaJok0yg.NeRlA-1732698327-1.0.1.1-hjGzsRbFDEIKu0wurEQFF7.tXqXtdpfx2mlnwdU0qBE" + window._cf_chl_opt.cOgUHash);cpo.onload = function() {history.replaceState(null, null, ogU);}}document.getElementsByTagName('head')[0].appendChild(cpo);}());</script></body></html>