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.sciencemag.org",cType: 'managed',cRay: '8e953573dba740d0',cH: 'H3mfu.iX_qZHCW_VHYACAOzFcdPw.mlscI.n8mzjofs-1732742915-1.2.1.1-bfXZfQEyc0PnyPkbP519hHZcsCCDL.KOhXlTXyFct80Ajf0PUV0risg_2xDoLQo_',cUPMDTk: "\/?__cf_chl_tk=biuTAPn5orKf72ymJgnhn1ralk_4W4ZvHuq4vroe7yk-1732742915-1.0.1.1-mBCrRV5qGnzQkq9IuAxSQYfEKQosP2udH9Y.j_rlBDg",cFPWv: 'b',cITimeS: '1732742915',cTTimeMs: '1000',cMTimeMs: '390000',cTplC: 0,cTplV: 5,cTplB: 'cf',cK: "",fa: "\/?__cf_chl_f_tk=biuTAPn5orKf72ymJgnhn1ralk_4W4ZvHuq4vroe7yk-1732742915-1.0.1.1-mBCrRV5qGnzQkq9IuAxSQYfEKQosP2udH9Y.j_rlBDg",md: "XzxW4x7gUuy6c7zduGtFmJvqRPMyJhnR9X5jAPRGHWM-1732742915-1.2.1.1-.igoBSXGpOMMM5DeN7OBHl0rw9YOcNqf2rxpH8B3taGywDS_WdunEEIYBXV95ROOXChFRhKtt7aDlInf_R7UsRVC9Qp2MD3diqtEWH3lBBToYh8aScXLP.mAAalAQenRynn.0ZASqLmpTLZwgiZKbVJKkkdr.skgLvkxjusr8IIFMsw.xEkFKxypuFn76Y_817T8Od.spO6Q3qskSNVUmzF34DHcsEZZbOKoOYb_S9KT4uyDdDNu0oQoCBph8_8DKViQ5MeLXPe72rfCEl1HjzZCjTsg9ccD1KjdvVnZB2soqa5sjFGwTexmMbKana7wQy7mbfZKw7OIBuXZ0lC060hqyllioiAE.y36w7Gn2QALKXqQdwJ.OVnEUdhpzoPFMQftf_iBfmHk6FY5A1R0FORqWkkzuhA_MPxu8FuWEho8G65zM3yvcOGq4gJXJAp2AK.lyX35EEP8s0wYxf05x9L9IuCNWOKgHGXEgKTADxoICo39QEmZiYj5Uf2qj9tUaby5XQ1eSbRWvk0u5sWo5vPoDBue3YGcVnOBae.xi.fz1cCgbHkCrTt3AjKkv6IIQi61ktwCsz2uC9Mi5XK.wDN8tfgHDWrHHJfEScxmO_ykO.Jjfq8VxnjLJA83fNNz5GEzI0NEbTBZYzmpvpUmCwJ1v15Sj3_vI8FZZ3ZNdgUHF64ynxOrmGLD20nCwBsxlaODJdFq4YUpHCO_PIZGWRRpKFVv4jMi0Hbvb1gRwuOuknqAM.eL_sY7a1nkvh5ydf_XBusA3IGWynpspf..luDdwpHcIKopruTMiSsxjIT3EgO3fJOzlRUKqhD0yCqWTj005x4ogl_mJWiVqhNUHAFPc2h20oc6O4SBCO3JkGnVYFg5O7rhBmMPQ4PM4c4OCFFDKxZPm7GfMebCcQeUlRfqyLFPJ71NilGTEvV5XYkS1A9oRWEjRTK.zggVbnPEStTr2rLeJw9bcbcZ0vj_sdCQfkUeVrwaEtYQrUM74l.wkQFktUy8dwUApMv7jIt_.ASNFLME1rccZD8uCci.GFo9VoKvT9s2RF.1kSIVbASlx2Er0JZtGp8tiYKQqM4ftCNXITApUx1y_ytlx49V8X__m6sTYtZNikR4RHXIdKGxCGcUZRwxxEexWclp9s6GMiL7whYoe31MBjqtAoXd3_NUJG6FYxwkURB8hc3mSjm9.VD046vSLtMGOhalQ.oUEZqfQ4Fpl6eOjcY.9BQaH_0v7R4gyoEat_nkSFW6nSXy6uCYbOdYm7u1tQQX3HLCnKwLLslridoYys0j0VyMLfQk6sJdrj0bsuegRcozJrNQkoWkj.wyftMmp1U1riH2QvvUUj5lu8RaWgJL.LXlwnvLAdZjrKQbo31KKGh3UdvQeOAIjvemtkNcN3AI6KeiStUA6weNd2_nBL6JQ2MAWvSGjTy.BpVObxWk3Dko_RXh9so_oEFLobkVWs04daVXNUnchN20Acz7OG2PGOwWMOF965MQK_vDqoTfwT1o9rwNtI9EmDArbx7Q5eyABS8_BmnPD2gVVIKekToI5Si8uQuH_8YKaqsMJaFdy1rCICIIW_dP41AUeBmWoVwK1NlN5MXLdi_i6esLw7n5_iipB2M1YwuFxoeHj06xg3KYOUv4CZFmSw1TcQIfjs7C8BwubAez.3O4dI8j4s6_CHT0MUOLO_u9GfOGe3ql9QhFl51hRZok7Bxf6ryIxNKIOSYD7VEwckXkTpEMIBiq6fLFZX6NzkyLnRMA7t3uPlBzJkx0A4JtyzWXjn1Gl8__.6ubK3ZMWY6t9MYLClMbydFGSjrz5KqvbMUaCw8bnslFbHq25kV35JTwbKnRlkCaM_YvTyIab57j2cQQhnC_lGml4UK7GyeoRTRiJjhiDAVv1Wk.B_gpVnWoyzE06x.vrQLnebGr2FpScGj2XvQhxlgqqidyinBn6lDlSPFgPwi2agYKauUGNKj2W7rTEW1NlkjlMoEuElESK8MYZ6rvuJ69i566tshTfkFrRDNM6PLoldv56tjSJvPjXhWb3tp.F5pXy0CZcGw1bq3zXDbzsfRJFkq_9Q4aL8hcuNuaxi6PTl2GlaJskF4Ybzu1Eaqphu.GBg9omurDmTwuxBgFpaooQJotUnP1SVk_14oRAV_aqWuHs9mmm9iX8dHW06FtdG3fs6CaSG.Ax2Gi_XMEIzxXnmRJWjiHB388ffg8u8_PR.jWJycRPG9cXM6laS7fypmQUwQWfjjtsbLmgDJG3si5WfwA.L3iTdEEWLLCbMWzaP1NeTLyg9edYHt76k.eohQ_eUpY41Onqnel1LluIKV9MyRI2J8pY_H1iF1BDxrEHmjxUyVOd4L7MfVbyGwk8c7Iujc.1fDeJzfOPgR69kwFGqfEHyVFubYdhsE.uPP35znTiNs812PQoLzqKUnM0yHN0RPIHnx78651eb50JzLpRLi6c.drfpfzljGBRytRsLZX1r4Gs80uZNxvU2ceoZscTSN0QCqp_t4j7QTvumFsIv9Zp_jTioywS5NP6nZOu_joyLGMMJR2Q1M.a8mV6bxh2pElllyBqkAbkFCUDVQh3rxjpyj8TmNTMv9cdxTSGntuY4urDKAZbHfznyXvZoHpk1y660WRBvT4_dZafnSz_n1MZaNGipVXtB6yZvVYUNnMIngbm2BZZwQwXBV1Tmm7kAcmGRYc00BLxll0V1BKCnkAiKZMbw71DBk1eXH6xnTjFFoA9gBjJHPZDzb.tld2Q9RvDo7vJroskVbM2wckOIXh1H59wdtwVLNTGf9kFRGw8tpdHo87AitLieaifDmee1jKnlf2ntyPWuP5y8gXWzGE.y..IiS.yRaZBc2loeTaBTc3fhUIX2lWgI10YJc6HfNVNndVrryBbh9ldEm1FPz1V6GIt0CtQRLvXU3QAcE",mdrd: "NSZVL5.imiRp9v.Ll9HaL4gS0Yw.TKbx47KEEV.WIdU-1732742915-1.2.1.1-36r2xk9wkSOBXW52qqQ0YayRNb5zuL.OO0svBEd8fzzn5XLFZ0u.rBloAFA7IAJGSZEyC3G0RfRwRH4GQbckFb_74B72tDz_DyEsr5kzo56kWQXqwfLZFcREDMPsOC.o6L.eVVmcdMo2qr_FfS0OB0YNjIAo46yFZPKQp2K7ocOtjvH1.kkV4DJARbLUbNX_fF768F9SstX6YWnUmUwSN7OzM_TNroQ8JCqBYG06OKuKfvL6hxV62plQNIDeXxUBR1pHe4uefhEGJyVVTPfudw92YmqP4tjcwCkfS_6yLYyuI2_Jqcj658Z.WswLwQGON8vOsa.YAJWe.zUdJ0qtNVC7RmUPj8vj4Crmu0jxcPyw2u3fSOeMggIg_U0ImCT_soDSiH_h9w.7yi2pd6j_nyFk7w8bU7M1hAmzvqeNrZG3FO4EE5dHjbWCZhvwya26MZn1HRU2NDHhZyDlslmc8StjaCvpgm7ta6UZlU0a.g8mE.z.ttbLU_73Uz4balW0clWJ0QNcC8h2S.pa7s.X_UxE1jtuhGxgys1O24.ARO0Wx1qtWWIT92tla3rEpV.hd3P5eGRgSCeqENYwlzSpzGEg9FzqKdlzpHSMt6lYB5fhSVz1xxm5WzqIn9znKiLDIk70usE67MWFEPc2fbkgaf5B7TsreEH2UH_sVgNvsFFxYSJgdyJdbgx3.2IEEE0vTwR__xvddUJYlA5lfQo2soiiErL4vcne7kVjvHhRCXrIbGr4kw18cObw.VuU9ngKUDW6sNeTugd5ExzXqibCFPyfNl81.9INwKy7WTxOazJCIjXiAPwQ2aU8KHTcFa8ZovkxrAcx06qLueuTsA6r_6th9Qz08bZ97f37cDyYLvRiscxeHeJx2WvHmEqZFnFRbSCoc_vYjy6zmpJh_z5Sg3mhbTwakj._V0Hw6K9kd72_4cq4xiIECPLtdzLTyYrWTyPjsGP4mSms0zEvQfjKBfxkGjbY.VIbzIcYknuc_A.BIqUXx1NMklz4N7RL_R00WZsZ_GSBtMX_QwddDVm571jzRBuFkbu133zE5TpAu6Tepq0O8MHqXXe4AAp0zQbbxIGMS6P6OgFGNzYCvPQTmstsDCqz0xfzQFtwvZ61kR07chyP2dlywy_0oLjDTYa.nsKBZSyXUf2Cn71yKQVbLtgxypArPDuNMmVHuz0.dDdnRtYNH6G54SKsjwIxrsRBoGjnik1.aBmsXzgnL8jwpMS_F4SE3jOXFucduv1b8TEPAqzyOQQhAr2JjTd1ib3qTp9cHH6oRfS77gQUxyMtnyzrW9b0JjELeR0ddefnIGNZHzPAE0AIYd18AB213u3Nke61XNjIM9kpggWRY4GEjOsvNClq1r9H7qhushnGl_F8k7UJZXG1jof96NMWm__Q8.X77ruZz6uAHPBbPYiL5sPNwyxtEDL5cYoqVUqStsBLXVWlAMMGGtwvztfxRgiytuX33E87e5OxMglAlJ8cPjfVi83dYPTOB9tpV4hyXt2_dY3ff4tGd5SdHG7Jso6jUbVnoS8dxd9uSJWeLqCj0sKUBcYq91ojFXiMdm.m2AmD61BBzf2CRhLzHfberRRX6SZJrNV.bHFA6ETOShNSv6EwXNNWhPl7iWLYamLM97YyxUBO9vcfTwT8NOTu7HsgQ37qxktcAE6Isgl8arRcCqGlxGssbZQUqcygy2a0iIsVKA6GBhDpKW7nlG04tQ3dpYRoI734ObiZDPDtqv8KRXe2rFrq0MIJQLDxEVNkNrzxOvlGUe2Dgw_kd22MJqRyih2LCcwV9hSxTXuGlGfKJWfGbX9PjpUnd.NOJBg6rqTuyoE5vIwJAc2mpTV96a0c3VKLI8MFW80EJFvU6CVTS2U.4W9WqXs0_zJlswbFZWY33.0atYB4b8EkCKsD4fnxyQs2pQrNVdRxTTBbJFe8Ra4uyDkLlqi681X7hml7b0A"};var cpo = document.createElement('script');cpo.src = '/cdn-cgi/challenge-platform/h/b/orchestrate/chl_page/v1?ray=8e953573dba740d0';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, "\/?__cf_chl_rt_tk=biuTAPn5orKf72ymJgnhn1ralk_4W4ZvHuq4vroe7yk-1732742915-1.0.1.1-mBCrRV5qGnzQkq9IuAxSQYfEKQosP2udH9Y.j_rlBDg" + window._cf_chl_opt.cOgUHash);cpo.onload = function() {history.replaceState(null, null, ogU);}}document.getElementsByTagName('head')[0].appendChild(cpo);}());</script></body></html>