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: '913796891be4f902',cH: 'zyfttr2mPqC2CGKgu9MqWwe0rrexvuLo.dRFTHMLFtw-1739814293-1.2.1.1-O4ZTnWcB19ybRUBpZwn8LR2PhmCXx2Qt5lf.cWqk4TF5dQZXOyJK2_Izlq.jkEjt',cUPMDTk: "\/cgi\/content\/summary\/319\/5871\/1750?__cf_chl_tk=1rCAbE1abVeD6wEDJCHevI7moTXYn2q1xEqRS2fxeDg-1739814293-1.0.1.1-zVM.LTXZ6teE1nbefK65alsJwr1z7TLAL0LOmwFkwpw",cFPWv: 'g',cITimeS: '1739814293',cTTimeMs: '1000',cMTimeMs: '390000',cTplC: 0,cTplV: 5,cTplB: 'cf',cK: "",fa: "\/cgi\/content\/summary\/319\/5871\/1750?__cf_chl_f_tk=1rCAbE1abVeD6wEDJCHevI7moTXYn2q1xEqRS2fxeDg-1739814293-1.0.1.1-zVM.LTXZ6teE1nbefK65alsJwr1z7TLAL0LOmwFkwpw",md: "FZv_VFMAQeDd.ltgKLOw4EqcwBcogaweGB5C2mYqZNE-1739814293-1.2.1.1-ZDKmxAYUs3luCaeIUU3_sheLHNG3SQxd87Q3tyhRsYJhJHeTIBXzLt3ooAwgc59I91DtA_Ex6DiLS7PJ0SOtmFDA1LQPErCvM2TKosUXKutngPbdQVreBa4B8lD2wpUMvOFJcaPwB_jAWsO3dPWSQO289e.0c03MYpYAEq3lXIRHd3afS89NUBq0pSR.AxHwSPF36aHrZaBeP.ahdDL1lr0m25tqHXiGzGWZl4TO3NK12567MjDtaFiQQHVYeNPHmZwTRONBB0VFEyLUikI3urXws3t3hlBts4j4BV_EjztFDUtIFK0elQL3yG8JWr4UgMb7cE4G7RuVMKmP6SlhxlIAMOKEpWD629YBPBTKVYzKNP9_7iwvhHxyT6W_wHYii7y_tnrrYD4NncKSYA9fhg_D08kKBg21f2VgVlaxw.XK6GVnVtVrPNl7vIQD4brWSYqJTdm2f0kSLxEbiOn_FV6udFXEDZuCa_6IKuNdfs.X_ja3tE6sqy7wK8qKW5YsuEjf5ePwgMUwP8da4zPT1cHBhsV3x_PqbkRjyj7kPVtUpOHuJh9nc6uaM1efxhOsPazuSk_5JkBQtIbILMuN4HRc8T402qQp0CqvVjqoF3mb09AdL_hr9ii_swNct1wE3Xs2U_EIxos1zKbkl0arXklCQ0UvZKVGiyN.TE6ZvMCI7NOd63Uh15PZgH5S6QOcwZBaQ4WIkB41OvzBJJhATRD9dJ4eYPxe6EEJ_KBc4T6VsYUtkSdqD8kiI4jpxKhDgGYmcWVNtmqIyZe7bx2F.2OpVDHoFLBPuJTpRdC1MoJG0e5mTvMSW4acaG1T8LPZL25xcTHXG03Ucj_tz0WCUZBxWG4jOKi756HrI9sZAnE.JPAsjMa81vVykf8XQlD6NqwWS.2lOlEEY562xMvEtNUM4_VVO5wvooAtUA5wXLCjkLR08ySArj5_LuW5Pf8X_8FA6HxvB0_NKve7GWRo5YmnXB.bJP7TXdZJoLskdtHCERkXMOluEtN5whuL0ipfZLk_9t79DNP6ziTojTGWBfczfXnpEVuDNWUj3FsEZIYHCuWtr8PJeBJFoUWRNh7pSaMJYVVApaY.2tGUHaWt5r4vPeNgdFbnO2Xut2Oy.hnkZ3ZSfwb_VdXm7TKCXz3NjFBdJ1lrUUWvTqLsxmY2TFjWGsQ0sg2PA5ul.LQaHNhhLxWwIoqZgwySegT94U5.5Ck_G9DIzyD_vf2cTrau_CRJ7LaxQLLAZDwA_8vNaWEhPMPU.CWDwpyc4lKD6Nu3cGz6nyQJ0vWRDfpw3QI7Z0477LJSgLWkETbQe5N7m4KKO4YJtTnQvfR6aF38D.KlbYYSB.SjYcHaVbg4tQtJe6kfBlmG4x2BCBVx5b1CzH9qscTI6jd6Vc7Ba9xe34v.sPdzxSKgSTEFiuCOFKDU8OZmFc8_NIS8rYpQcSfMUFvNyF6a9iKWmfFjQr0YI4t12gYjb0D_WtphvM0bndRmBvc6WbPt4brgXxm84mEHz4NBiGEnqTEac51RSAw_fQQ5UYyaHyu2RXF0kL4l6E8z5JJ0Vzbr.dCvGLQLAqanZh4cTW5iBEBePVG2SeAFzSCwFUToBowg8Dph1NY_CdILpy3LgAjSddC6g2a6q4nipMh2axsShVpSRVIEpUbdjROxOuuPzr.ruffP91ye1d3oJyA.ONi65BwowYHqo7or9dciT7M8y3POn_zpeY4NRzmq.fZN5vybSGdIAnf0UiDdFIpWr7Fm7CCasAtPe8km_nU6P3T6IKUPdtVzlTvY6.yvWnMzi0cH7w4ViGCSXJC04S3JL983p4X_2nU1ikiqaNTOCZNfSz93AnbylBEeyhHnnWZ7vDgGB8tca.Z_ctSGpRqbhqzQGYyzES6qnnYN6MJ9j29UmObdqbSQH3.Sb3DXBytTocrDthvPZFH2K7vR2VuSKo8M7eUohBQ6CCcBG25f9baS5zg2r1TtJA3QnMCQpmRDWwnZAxZDXYyWWgUy7UIrM7D6cGUJjfm2TTK1cCl014SIhKPnRPMSGJMZjjw2zMYeTt8WcwPoVi2V9TAEi7CIV_RpdemjcRgCLP.QbUrnpkkmYQBKVbZ73ZqzvehKRP8gz0W1LWB3EbBfvfC.US0Dyj.irQX8lx44XTddfISyf3ZBnXQNxT8cZ5TzVoUbsYNQCvtY.0XHNXhb6MeN9Xn4lyPDlK.AyXwJRZYkZa0UOCmjA.QIa22oN9XznvBCnaskqocyS8Vqvhpi0JOYw41GUvve0vI1SMJN69wegMvDbPn0g.T0anqMmmk0vvZD9W1nCmNIoeWQzlrFp4L48q5CJ9X5zH0aMuIMyshiUI4nqvlI.wmBliccklpOS0g2OqfPb6qzojXWZelEjnWfylmCYYo3qUwAN8SFiHjzgC0zu06oCHXkg6l4rZNng1wzp_VzA_tioc5ja6BlxPleM1ff2qfBb03YRkqBwkmDPxJsMqbFSy7K_mHsXvJE1wNoGdXgPm7XFxhO2bPI44JDzZ6I50x87ejIeMDnGk0tzikKvl6LjNpTsIuEeOgMxQNZvG185VOC_A4LHGEyA3oPExeXL4Eje0Z57aNEvNBYw9Y",mdrd: "7MGbWUxXgPkWVyIJa7If6UV_VBk.Jsp8qQaaWP_d7dw-1739814293-1.2.1.1-2uSsbdxh1.SYp9kXo_uksGcgXs4gbSp.DL6a.DtQS5BbBeoPZDSeUplcHIF0t0zrMTenxqI5.MfQpKvQ_3TSaotZIK03JsnYT4pudEFTq1oFLUSwcTjrd_5_C9CE9uPhTuCVAziFA7aS8WW3BSlIhWPT2NdrvMhLmbM1uAuoJIU6_k7o4cv7ECsMSaG0iGDyh8qptpIPRPKR41a.5a3SB_IQ8P170Z9ZSpdCouZBP4NYSexnu0KwO6N0GZkv3AycnSBmAnifCo9c4.a9cVc6SgT7e7ZJNWgYg.K4oa_.VEj6zDnZ0S4iQBQ5x9sPxrMejlHazC2H.mx3ttAJjuE10uFuL.Jj3PUhBANZGfn49VFIO4fKBmESsV.8uB_Q6YIZZhLzgBfgHUS02xdVqoprQ17sgpMqhFt_QIAKUncRKaXBnWWpl6Xl9p.P3EUxjbY0ijHzEiLlq6G_nzbPCMnarXHjgkQXvxAxNoEJswRfFUhpIKZ7bGPPuQy8WVZvY4aXb8ryF_0_FMSbepl5IqTURADU3Of2kypB7lOJCwt92NT90AykABcn5ovWBYwTjADd4yJHvB9HqqywYJRfSg4K5eOPGfEcqskB32zgOzBFqCBDP0N3XuihFdVBwyyYjuMEmPGrpvlh3NiE1n_4sK1wVx0WUHPvvjAC3Be6i6VvVLD39Otqf8fjut9qSnId0F36"};var cpo = document.createElement('script');cpo.src = '/cdn-cgi/challenge-platform/h/g/orchestrate/chl_page/v1?ray=913796891be4f902';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, "\/cgi\/content\/summary\/319\/5871\/1750?__cf_chl_rt_tk=1rCAbE1abVeD6wEDJCHevI7moTXYn2q1xEqRS2fxeDg-1739814293-1.0.1.1-zVM.LTXZ6teE1nbefK65alsJwr1z7TLAL0LOmwFkwpw" + window._cf_chl_opt.cOgUHash);cpo.onload = function() {history.replaceState(null, null, ogU);}}document.getElementsByTagName('head')[0].appendChild(cpo);}());</script></body></html>