CINXE.COM

Just a moment...

<!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: '913dec55b955404d',cH: 'pmnKf9yABAmpX_jfItADnrfGQhyZc5Ay9om4LVz5TMk-1739880722-1.2.1.1-yMa4PMhjepfV19DfqNFneuiyvwoc0yzTrwdgRp7Cd_M_mwgtGynA30uA0vTbxoxi',cUPMDTk: "\/doi\/10.1126\/science.246.4936.1417?__cf_chl_tk=JfQze.lyJWhD0rt3TykbKSy3dQFvZ9EWZz2ridax81o-1739880722-1.0.1.1-0Ul3B8EYt3MtOZG4nA.qaStdac1__DH_Ss4mjtSgiGw",cFPWv: 'g',cITimeS: '1739880722',cTTimeMs: '1000',cMTimeMs: '390000',cTplC: 0,cTplV: 5,cTplB: 'cf',cK: "",fa: "\/doi\/10.1126\/science.246.4936.1417?__cf_chl_f_tk=JfQze.lyJWhD0rt3TykbKSy3dQFvZ9EWZz2ridax81o-1739880722-1.0.1.1-0Ul3B8EYt3MtOZG4nA.qaStdac1__DH_Ss4mjtSgiGw",md: "P_AT1bCdMQblaR.JBR0URbjlVQrY1zKXKqMrVxM0Q5o-1739880722-1.2.1.1-IltqYG0peyczADJ7stML9JhlpsiRGjGenXbMmq7o_juu7C6DmaWMew0vx0khdzFpGXLm0sj1auZM78gHvhufSEYGja7ckhwnifD1qahXc6L3kH6o4R34h_vg9u.k3ayJW53IFtfakHhKRvJ2UqRhMej40ozZ.Z1VW70n048t.THftnk.YcMa9B4PEYYfSmF21_NySmJVgvd5yCVGNwZR53ByCd.mDLaiJ1kX9zumnI5ovppy2bFwcezMjgY9gU3em8Bg8j7xZ4tgVUe6OJZWfzRui_gg03dkYFWVjzDVeUk.GqTqtOiQ0ptyX33pvmBeL4FrrCynWrs60bi3RiqOaN_Cn2i3oA_ghRUiCxGFB6HNJV7F1o0Y6MkBjyGBitqc7FSn2xOtjiJGQ7wGOx1vdehF9V2vUtEHhc.1YoQJpRgCHozuwxrBi4ZIiy9Bjmw54Z17aESeqQZ8DbHSTb9X9kuIGH9bCR6skHhq1iuxm1p7vZZu3yKqa4Xbr0E0hXKjtwtB07gQjBxUlF.EmGmNirPDXeundqxorcAO5U0U_7gx.28jjf0AQS90wUzER7CmivmleCh8AbdVEhXctftuBFkVTZaZ2e5SYyPKnbYPQ36xNGw1TgRpzzamzJd6XHedB7hPox5M7XMevSVmJEZLCTC1dl4Idnq8S5CoEij_P5Ev7pJl0nsMrcJqn3ndHNZUXvNUDTxgAqTl04KMPC1TO7ZSUPGhV8hsZfBVSpFWW16TxuLO3VkNczY.HamX0JuJntYAUQTqjEsr0jFuwwKgfF_izevizU99SB6gnI9mxuQ6xE1CvfuAhGtLgbdgJ0yDoeS0r.svmoDrNBSaTD7Nq8IOfJQ5om_65q4ctItoIMGRI1N7P57RV4SwwpCvpo4FA2lDLV_6L8N7979G2JZMzCwMsNjGHTpbPXUnM_lp6Th_fnCHx0ueqIyyFdkyenGwV.I_6ty1ZBsvWKIhECWar_9cpc2JNSPjQO7_Q.6DBh0HNo05FvBMz3.iPaLSwidfD7WuoVG8tjf4VBI.kef_QoNmrIgmRWaP1sbeaM9TDnQYiXuogR0D3.P7CwddmLYg_r4hyREo2ljjpY0oyg74DUSGgd0WPHq.FKEcUoW_C5fsRN8ej2DF0LqtbPbKFol805D0ucwQIorfFkvomIw30BYh0sy6VvmbxMBgqM2CvXc18HfgolbW5OrYgieJ_qJZHHczaLSCSRalyBt2Uzc624f5c11L0QjKv4PkFOFNjc.Hi_RUV8pys2WkgVc9FK1k9wNAYI8Wg9XF.zCBOxYWj4l6AtIDzyQescopSwSlNtlCdWvma1W.tzNmNufsl0MfYraGeNgdUARBHdUgNqFdBekQ26gtVDAQwyXujed3oVuE4o09TxlqN0HSnTs264q1i7TISOPuY.x8aaIm2p.nfMg7uzsvbZR4fYgGDl7EA5BRL5nV41m4Z7wrmZJnwnZGCZP1DxEOeczRNGjMuEIsmhOYleyBXj3A0Dpf8zw5_H.dXPeXHeWzlCoHDNFYXQUrffMOYixalyeCBO5XawjtjGixCSKiO0k2vs6rebH7bnpvVKG_G8hlthfckMBz4igUlZOTf5ddZXcShbETWPnyimqDih4Q654rdqxf_oHT6qvQ4n9MZkAL.GlvmPabqIdQVc4mMyFJ4hAEOEXIL6fCN_506MozsHgjpxeMvEto4q1QKF1VKmnc7IZs5YQTQ_jQtGKgaT_2N0st4cXw5MzzFDmppIHgY4HRui410g6REjiiGhNMiXcYEK7LY.6zENLhH0A.JPsB1Qb7Gcyko6eDldwodbg72nXjT3GPdTC2miA_YG9eXHwJS1uAu6J3SAt4IuM441aYgK5GHp.DqDgdM1bOnGKFHmi3wT2.jVvnf1XgLtmUWPjr2doOzbq3H2i_qoV2Gmzq.7pvd7um4ujkEwQTgCYQBIl.M7xDen5Bdx60TMUhShW2a443x39tXBmD1gms0Ke235VltKcfaN2mPKn0aS2KRbq_QIns6k7qSDjd_3El3rZmyk3gdWyJgQDViy3qf.vcBYrZkwZ7pCHBbUWZFM4R1BLNITrQ7gNAz0hvOwOynVLDbo.jwmmsTn63yBKiNotsE7Ia0NNQyY5XmwzPDMafvwAlf7hcO33byBwhoOmJuzEBPnIdH9kVRYXI4lGgVL6262cogvDadVp6mReIf5qjsZEEI6YutFgulNOX.bW3EPxwtIhwbPelgq_dBOWjocY0P3Db2PpAyefieIkqosXCbDZ6xeWGKJxwmrnXD2O1I0_y6iKiPIMspLPmEtgDSUKnqBy5e010qj_knc_8OLEoBHQif9OqEb_0qMA_OxReAy1YZMqxGLFTlA..DeubtF7RXnTSyzOmTf_NEAxB9sZa7jeBLBgDGMBJ.e62f9iWwhn2BECNjCAPx7bg3WIo6NLJW_QKSCQgX24tPgL2mL.nMQdsnY_UF0o9vFAsvIDk0cmrKAYsYwrovPKTT05GBjfOAeRZaWMSWQRl7ZMP1yJ2ZsP4L77GssnA6BkgO97TBOvkxG7oGKoKw7f4btPWnzXjhyd8kDPAXIO2dwgrirS5Utn6JBQeRTNunLA",mdrd: "ixmzbdRa4GIjgeiqsYBQn5zheHlY1WtuLy0scuJflkE-1739880722-1.2.1.1-RcEb8GNWhXJBRMQ3fI3VPrUZOnN8z6uk23FjTyxWQx4DeoS12P5ie8uuscU0gIUCK0vz82btvw.fS0_DSoezsio89UNnjAvuV.oV.9D2QLQPW.GeJTa5b5kSbz.rijlZexNiXrOVwCrfPAddCraIuQ.B5otm1EAr4udC2n7n164_aaqUdP.Kbegg8mDSjTNnswU5_3CGeTiY.rhe4tjlKEbrh.R.RqXCkjfah7cLkrgnQYkV5XCDzntbRhfCSDuN_3qXcjnSzs3_ZLpbHdC7ge7e4bp9eORHgIdheB4YYIRGn1yfHiG6Yx0vt4qCHJ7.1gkDvzsuvRUYCi6tTXpAUy0mReKSDUcJk1HPvSVJzEom9.gYEXsPe5maozQzyoI6QDGj_vVNyeU4m79uSZbr4iGeZyDdf4Hzi17.KYqfLmf4RdT9dOZBDHAZIZCR8VdQNXQaztu.M4G7VQnzBXnPEQ_oJmc1Z4.ti3VgjJPYV6HiOyKt7YWYfCZk5tyki9IPzw8_Tnr_LfpnhjnG4_xVWieYJoj3zUKSVBiZtCX637AtEYMSXYrCeFHJuAVdcbzUf66OnXg8FaG3BOeyPPRX1pIwtP4.sUZYREjvcBc9QZcvG4BVLH4UaUg7LSyQpQTqqtGQvQPvzutGZRbOnLmPDT3ox1jWchbmcf0i0CbiMi4z4YznqzOSm09Dpmhc95ClVYt1vqtdpyAw7YpMkIb1Euj.eDU9LcZgSAJ7Rh7HnD0Ojx6DwlacwaXnaWztIQiSltnVOrQwA5w5xbZooVhtjntK3ZcvV8SRLVoHBdFaEOTRbLC.awHmB0LKyCRZrfIwmbkFVhSvjGAX4n7OCz4C9gSnd.MQ62v1zwebL80crEjVhdnIStl5Rst76HXZyzEKJ.AcN52bUu3UXhyZajpMj13rRtpA__OOuHDC6YO8TccKgDnH2M7ik8ay05OtnqhIUR1FpPSTf0wmavZ245lTh9h5Tns0LiqUJXBmuUTB2Ya50.hu4Em47FBa.Nt8osD2mBQcP.LL83Ehyq5Ze6eEUnjYhzESjBfAKFv3dMQLgoBIErGib.ccIi5AljzPStcsfBdP3BrNGG.x.bD9Wl4JtkbYSpszkQaMl7WxUbmZk3drCAQGyjVCJTF66FZeQHmBzsjUGMUFn.wRU61dSDBK4.w2QBZuMIWnY2MWK7lCR2GLmv8ZZxVZDMVJt8.AHitsGIWr_WYIVg9eUFTjtnLc0t3XXI8evWVxfwXsUM6qyBT.DIPr1vLGywe_dDuDhkw.B8k5af66W3ONo5d_ZDzL921Z1Kh5rZY18zYiI6Ticen.lSlOy8JCPbmW9Rsq_FX1dgTmefo4cxgXFAQ6tsg9aAGnzHMZmWxm1F5dFtD_RSVrLrF7Txfflyd1LFo2B38UDfF.QmzJt_6vQgturQcIQKb.vnaj4Q8OR8efzrjZvCcI9nBunBO9pzzDQiaeiGj5GMBw3dzQ2I9Hk57u2akwsLza7Tu8OzwR8pCckf3hFQ38wzA7PGYyDBmKjedv_QD0z5JIH9ovTpi4bx5AJzdyn8O1.c5xL6Focszc49Ud29WzNT4ykYn92gaZ_H6q.U_4.y_7QsO0KnYXU1zrqv_Zo9EL00pKeh84tkOexmKzsITYm9FXCmaisRRQTF_oi9n8nMNCYgIdBnRi5NaobmQBF7yMunI6F4RtP6kv5QVEH2QCsJu9DsBe7S0wA7YAwsigHEmy_DwlfCYxGjgMt18fL9UXMt6dyJicyyBbMAMzfPf1_01OajdbG.P5Zd0evGMVHsCzkSz2x6HkUgy2A4wp_Neqg5r_AIinABxIZdBB31df0sqyxgaWsI08YFoPSlPDxAQGFzdxByYhncGt0_g9wZ.zhxdpl2iOt6q113yghI5_RLcXQZt3MdnxFrvNy4fvT8NP4VJnklQOOnRatm2PmT3TE3t6h1tStBlmsZopoHE"};var cpo = document.createElement('script');cpo.src = '/cdn-cgi/challenge-platform/h/g/orchestrate/chl_page/v1?ray=913dec55b955404d';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.246.4936.1417?__cf_chl_rt_tk=JfQze.lyJWhD0rt3TykbKSy3dQFvZ9EWZz2ridax81o-1739880722-1.0.1.1-0Ul3B8EYt3MtOZG4nA.qaStdac1__DH_Ss4mjtSgiGw" + window._cf_chl_opt.cOgUHash);cpo.onload = function() {history.replaceState(null, null, ogU);}}document.getElementsByTagName('head')[0].appendChild(cpo);}());</script></body></html>