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,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIzMiIgaGVpZ2h0PSIzMiIgZmlsbD0ibm9uZSI+PHBhdGggZmlsbD0iI0IyMEYwMyIgZD0iTTE2IDNhMTMgMTMgMCAxIDAgMTMgMTNBMTMuMDE1IDEzLjAxNSAwIDAgMCAxNiAzbTAgMjRhMTEgMTEgMCAxIDEgMTEtMTEgMTEuMDEgMTEuMDEgMCAwIDEtMTEgMTEiLz48cGF0aCBmaWxsPSIjQjIwRjAzIiBkPSJNMTcuMDM4IDE4LjYxNUgxNC44N0wxNC41NjMgOS41aDIuNzgzem0tMS4wODQgMS40MjdxLjY2IDAgMS4wNTcuMzg4LjQwNy4zODkuNDA3Ljk5NCAwIC41OTYtLjQwNy45ODQtLjM5Ny4zOS0xLjA1Ny4zODktLjY1IDAtMS4wNTYtLjM4OS0uMzk4LS4zODktLjM5OC0uOTg0IDAtLjU5Ny4zOTgtLjk4NS40MDYtLjM5NyAxLjA1Ni0uMzk3Ii8+PC9zdmc+);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: '8e9b26cccbe09cd1',cH: 'qiVffGdL7TIPYOworjyG6_XQIJI2yCyqNr5ICQwyQYw-1732805229-1.2.1.1-VtdDqHVOespEtvs79So.N4K6.AvSf5jVdMURLyO8EAJ815wE.zEkMDM52JRUGD73',cUPMDTk: "\/doi\/10.1126\/science.318.5858.1855a?__cf_chl_tk=Jmq2gyv72VmM1tB2eKtDtT0vvJdocbwyBT8ITb_bziw-1732805229-1.0.1.1-JG7WlqPoAUug9zIdrBeCDDEg7HcqJYGynHsL3wcfxOM",cFPWv: 'b',cITimeS: '1732805229',cTTimeMs: '1000',cMTimeMs: '390000',cTplC: 0,cTplV: 5,cTplB: 'cf',cK: "",fa: "\/doi\/10.1126\/science.318.5858.1855a?__cf_chl_f_tk=Jmq2gyv72VmM1tB2eKtDtT0vvJdocbwyBT8ITb_bziw-1732805229-1.0.1.1-JG7WlqPoAUug9zIdrBeCDDEg7HcqJYGynHsL3wcfxOM",md: "8XpdGc0kSWyLb9Q.WPl8M6SaX5FMmgfsaM52nRLxdFg-1732805229-1.2.1.1-5jEXKrxqgMB14hXGLD4G4Jl1sOc8nCy3FQ6UuTIVydEErzZenchmrjDan6LLsfZlo0pv1AwiXxOP7yMLEDqeyn6cfmJGkdbq28rZ_Q8tMngE_sHA2G9OzzLbazqdzCkhqFeREFO5F08FVkIQeCYxRG7VZFP8aIbg_26Ua8fBJxz_arz_97SA03bBVfEhqm2pj000yevNGjuUfwmI9NDWMQ20noTYZBbGNSSpCt.Snd308d5IxTs.Os1yDKSFMjIWhcUoeu3PGdM_xG0qbeVOmT_1h9m1wJ2FIF0X95yF.cgKzNl.yquF3GNhElYewNIvXgwuaEGqIlh_IuLAxI1I1lxWfJ4RKrBQLglvN.mZH4nFnjCsoLnOtimbix4MZDA6.D3MQyl.dAtjOFfGnPrVQbsAOdeM6RmPiiXwrAn68rOIiH1AmPJKNK3WJUAnKZgJWaomDe9KrUwnMT7pHmXmfSC0BGbIwEzcN7ibAjRtwHIbYukLFACDcETNLCMfl5KU3pu44iBcc8H7oJfG9uK9uGh1fAZuS2irMXY3o5xE.K4tlyfAa00Ef9VXUOuKtKeXSCmtXxveX10NcmduYG6A_VSYfKCeCVg8sJJ8o_Swr3XZHmINu5jPdChGC3lravE5fOZBU6RBraSHdEhSgXpaaGGoTVXYIceRywUuZsjJp.SveXvFZnFt0tAI_3.Ivk4NncYPeeNhVoheE6H2wF2sCy1LqOGJclrZNvIay_C2eESIgjySBIlPxqBq70iy_7OZMfpikE_rPB7b7763LqtvO9.Z2n_gG58fZ9M.H4IFycQ7TuKxU2Fo0g7Q9CzpQnK0mdGKCG.UrSltNjBGD2zJcgKCbSTiLSpZsTNKC.T3uNAFqAvhdG82adj8yNNNblQ_7eR3zgtKSDMCLi.P.A1tmONPyDOJCboV.hKkQCpDADADKEB7gm5rDrQN3LIhKmVa6WsBrAHzbCqmbsmXbp1ZnwwQRWbeaHEDJdyEjzFg4fVCUNuAoNN1QiWs9Pj8Gesw3eNNAOY_OLhcxJHAQQQ9DwjJ3pb7ottBU31Hqz09FnGtyOwuEqURCcP4oyExAUWrsLWFiJu7PD1zMLpjsevb.HRnKe8nc4tj.6YA6Zj_7PgLdVDILIxY6wgQF3ERSxJQ8WUow3oTj7O_Hrg4llyuWdhgU8AwlubZ3GvvJfMhAG.n0.sLqe_dhNGz6WgPY_WTD9sxLPC9RJqEb0YXEZhIhhaGS6X2aJ4d238xU.lmwMtesez1mnYZpkYkHACdA7KhEdPXq4C9H1M7BNE6KpvduHgKYDLgfBIBx2KdhwULyo6urOZlt7JdVD13HhixOgt90N_Pcr5U8ah8xbMDbOLYIE_0Ynpu6v_tPMyIm3hqNcw1ppD45FKjzY7FL3uLDBV.wF1wdVrOBTGGZXNlshsTfS_c2mIzdMwmRbiu6hPUPXcmUw81rmageDaTpomgg36wAo48TfC8nKSpcYT4E6AD90RXx4lTRvbrZXlKgSXQ_V3IQbeD1yF_kdy6oyAhpoKP9c3DCoK4ee_ve6YlklboyiFrUB2f9SMFasPMsNNoA0Y8bwIbIN5vNEsflHuoZiir1Yw36oOgMZrX0ZTKCieRXi94lqtzSfjXm322ZScq6s0yqrZ1zK1c2__qttFigCGpjL0T2.fhh6tYOJtHh2IE0PiK3uhX40_sfyLtmGz4rtlw6Utnz3fy2lYHulQ6mrlY7slnql30XYn3L45PLEreuznfQg7lbj6FJc4LvaQS2dZuqzhbGhIphRwoNPQCNcf5Q7lzcSFBnPFAyWTD8r7nH7s12HcAqR3LwTmYIrw7yVj5tXQRG1YTuNKoOQCDvwnVjwm5Fkik3t5snoWVRYiiHci0lHuSLt.gOd8e7ub_Va8VZTTZq0agbrCMo7o.nLsFv8MUjB8Od.Fa2xNXeVLGcdpjr55zGAIjaO3LVWyusBR.VzTfCS1zNBMjXFnRJA4_a2Kzy5LcWuEdprugvPvD_4lA0rsYYNfdHqEkOeWmcU2i6hLwqIwvrRf9bKmNEbZlUeiNxAw3EQWfIPaa_F4vVUisYHx1GF2iuGokl9HQzkIXXW3X8ZEnmzIKueOFf_iwBZTWL4Cr60aTLMVD14ZyUQGbdRNrGHxhIpLrv2CI2UP7uP2uGhrSDAhyzjp1B2nABj.OV2T0UMEyWF7PokLBXMd5rr.wkV588R_cN4tgmfFoC0zPj5Hw5q0WGsK0haPj.6v6NU2cGZxF31Z_j3L.OYIV_jlP1E97.0g6rRIKhw4LhOo4WzIjv3NixE5ONL2377bt6SQ.tBsN9e0NqcCTJ0yCR5ZQIgRgCqmbHL20uPxQKWqinrk82mCAOnqQVNq2l7FlJAV9s_gRf5LkCcYplid_RERycGL7_izdom6m8QqL7x9k1Ce5Y1apX4i9iO5ah8_JeXrAqQpE0E5.nGnmFftjzUtNI8kgJhwK8hkCJK5QF2GDcAvm3VjHNVy_XTNU_E2Q2aN0nLv3OTi3jCARCDNbcYyx6._solRC4yurzODhLFhiG9op0SjtYvVqus1lJ8I8Naefewq.E_vNLdLvC57BbCI6iiggmFJaq4KdfbfUV8mPcu9sndG0lkx5yItNAJTQknXVEQPoE_huLrTp3DeIkW90xMO_BfZa_OHYbkGAFIjqPuEy3SkH0v_517oIT21Y6VKa_RJSKGvnxJANOeeuSTPW.8ZMvvt2O19M9ztlECxqYC7rBQ84fAq6VnrKKbemaYbzuDk2bslIb6Zzjj_IByVP1SNmpTcl72H96ME7gpZQmuK6410.QnB0EY928ZIXLL1aYuHITnrZVDvNEOX4cHQYue4f86Rtg50pWxM1QqHqgGyunjofqlVKVcytzIgqjHrY9sYhUg7AunJz7hcAflvnGd6X6eaBRyY5QNRaH90klCQRGGc5IOd5paf8RixwP249VnwKhoLl2MEMheJuWHc_aajq3yL98Oo7xF946h0VepDkQbeQDw_fY2m5",mdrd: "YwMWEpSCgHK514eredPpjapNXRpRCss8BLfElgqiCm8-1732805229-1.2.1.1-1oKwfW.YsMQxDPhHmV3vy2RAwAD4Mn9jWRY3.hMMu0rV5LW7loOYkurR7vfF2bIyYFQF_mOHowaHkr2H8EgorrbAn.Jvj9WCVcF8GoroQlbGJrAJFsqSJv0JSqfW7BwlYfBGAlbiKpb1gp_8oYBkj7r7xOHnODpSL71MWUKeqREWPvzrWB9G_cmzoj0H1V_L8jiYJhfcVyxllAh4yNYzrZtxAUN5q_HuGpVUnPqt942TOtJvP9.74036nsZGBKJjvekBLUX4IkQnsCofH5Wrod1yVz.2YDR9YdUpAUVvJAUBESGyqNtMi2nm38_1BAHK6U8QFBL_8cepamrjmEtWGhrkZ96RpZ7I9U_hbAGh3ZYf8M73tNsXsBD0MRWJnXgtodBDR7xRuzyS_Pa5LrlV9uv2ri8VKcctxBeRR801TlH88FLLHir7wDAfXCCOvf2ra1warZ1fszsGOS3EgL50B9k2Xd9PeLLjMNH0JkB8X1_nsWAI07h4dHgKmRVsRBM2Qe.r6lIwKN4gwKEecMdHK9_bEhX3xqn8j1ghtX8gdYatU.OenYhiAbI5aGql62g6WB2yy74wILV.bGjbf9TukVS9SkZCvtgPNjdTEIlAb_VXp5unSQVrR3gWW7kASErhTGFQVEI4qfWDE8CLv1.dWd2_U3m7qrKWD3DYDsrszdOwjiox4Nn_3dDI8dbhQdC5qTKhZ2ut.0w9jLwcoHkIwzyfYEtN6F3J3.VxgH9cS8EjDXfzDBVCDe.ObBraoEE92IAIJfOvoiFyLz41.Xuv02bBT5PQhBJbjGqNYRsQ.X64j8R8vQmZzWjFRuVUT4PPQr2Kc6yZeivaBswKCft5n.bvxWBfNoD3uJb7M_b1mUXvjXvZqEgkytp6WHzGS914J1p109opDiSSeJJF.wkv5V2EHfmhCcX_BsSPdEkkE.XDJ4luznbJQ5coX46l5Lobsh25TwCvjUvtYCfRP69tG24jBVYq5QRU2bc9zo1BFsFK6G1ojwTe1.elB3mEOt_mWjZBSFRF6_cJhBCA1Nv5bJG_TqErS1gWXyXG1n22ivslm4yt8H2CbTTrqPDaDPiFLWAquANxnVbD2Og3ycwUtx1D9lr79ifJDa1pmGKSo.Ji_MOGYujSHjC0_cBpMCYb8trUDPqS2hN.nGi2LTY4Fgk_6_a8uyTRukF1dHVbRD_pOg5wrZKl6dM5R8vcBbqN8tVwcq9fsJDEs3m_8xr9DTjr1HmkXwaUc5lIJ9eqdg9d0Cx4n9jSwsUA6IMOoAoazgjSNLCxSQNqK8iOLy5aiJr9XPxDNzlc9K9mbCJrLlXcT_aU7.8EuY520dQxsObV5jb2_b4eMpos8sSYbUdwI3fnEai8J.7NjxdGng44GSUFIFAtKnUuiHDtpREHisl0b8mT7i4GRj0sTpoM9SJNxoZa08Mroau6fyfTeT7xitr7JDZwexhH.XLSlnX.1.0PwnVP8g7w3R7QZJcjJsE2Gm2.Jt.ME3es6K1oVgEKvvoxLd0SnECqwspb0.zyMeFjtIhlEiZHaKrrbxNiTqnU47EOYrjBGag9s1VLpcBZDs5tXnniUjBiw4jRP95x7.42bOJZ.Lh5DwWmEi9Ro1kncoo5Ra5Z2HNPML.qEWBQrqglthW4xNt3pC8R77XZS.AGA7ObXjHNPb2gan8LCHYXshF5IP.EeYw6gzKFBh7PJiCMkLyt12lgSsziBLklHO7aQOgySEz1dGWg7ciX_9IMBqhXdusz8DNfCrBZ.vyg4G3PT9NG9k7M_nNoSIxlywoT03Q2IbVMpXaSPERrqD8Yj0nE_Ketb.svntLBmY98D_AdFs9IK5ki0U1812Ry0BaeSYWh19RXIsTJGofC751fV8pp0Ivi.XangqRmRY9ogYwLWqCZrB2r_ZFkfTejDRqJ9pvKaLDPDPQFHmA8KPRjZJnAeCsoZvaRRXplpQq6fyJVQ04yDHmQaketzDdp2rAZ"};var cpo = document.createElement('script');cpo.src = '/cdn-cgi/challenge-platform/h/b/orchestrate/chl_page/v1?ray=8e9b26cccbe09cd1';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.318.5858.1855a?__cf_chl_rt_tk=Jmq2gyv72VmM1tB2eKtDtT0vvJdocbwyBT8ITb_bziw-1732805229-1.0.1.1-JG7WlqPoAUug9zIdrBeCDDEg7HcqJYGynHsL3wcfxOM" + window._cf_chl_opt.cOgUHash);cpo.onload = function() {history.replaceState(null, null, ogU);}}document.getElementsByTagName('head')[0].appendChild(cpo);}());</script></body></html>