It takes a spe­cial kind of per­son to calm­ly debate those who pre­fer dog­ma to rea­son and who insist on ignor­ing or dis­tort­ing evi­dence to suit their pre­con­cep­tions. Carl Sagan was such a per­son. Among his many oth­er sci­en­tif­ic accom­plish­ments, he became leg­endary for his skill as an edu­ca­tor and sci­ence advo­cate. Sagan com­mu­ni­cat­ed not only his knowl­edge, but also his awe and won­der at the beau­ty and intri­ca­cy of the uni­verse, bring­ing to his expla­na­tions an unri­valed enthu­si­asm, clar­i­ty, and tal­ent for poet­ic expres­sion. And when faced with inter­locu­tors who were less than intel­lec­tu­al­ly hon­est, Sagan kept his cool and car­ried on.

This could be dif­fi­cult. In the audio from a radio call-in show above, we hear Sagan answer ques­tions from a caller with a clear, and rather fool­hardy agen­da: to best the astronomer, astro­physi­cist, and astro­bi­ol­o­gist in a debate over Dar­win­ian evo­lu­tion. He begins right away with some ad hominem, call­ing Sagan and his wife Ann Druyan “true believ­ers, who are no more will­ing to ques­tion the the­o­ry that you base your beliefs on than were the min­is­ters of the 19th cen­tu­ry who you reg­u­lar­ly crit­i­cize as being close-mind­ed.” The irony of accu­sa­tions like these should be obvi­ous. Though the caller doesn’t announce him­self as a cre­ation­ist, it’s abun­dant­ly clear to Sagan from his talk­ing points that he’s defend­ing a cre­ation­ist par­ty line.

Sagan attempts to answer his first ques­tion, but before he can fin­ish, the caller leaps to anoth­er bul­let point, the “gaps in the the­o­ry” or “gap­ing hole” of “fos­sils in tran­si­tion.” Sagan press­es his claim, with evi­dence, that “the Dar­win­ian con­cept of evo­lu­tion and nat­ur­al selec­tion is pro­found­ly ver­i­fied.” The insis­tent caller again inter­rupts and Sagan almost gives up on him, say­ing he “rather reminds me of Pon­tius Pilate. He asks, ‘what is truth?’ but does not stay for the answer.’” Then Sagan, with­out hes­i­ta­tion, patient­ly makes a case in brief:

Con­sid­er arti­fi­cial selec­tion. There is some­thing par­tic­u­lar­ly implau­si­ble about nat­ur­al selec­tion, par­tic­u­lar­ly if you think that the world is only a few thou­sand years old, as the Bib­li­cal chronol­o­gy would have it. Then the idea of one species flow­ing into anoth­er is absurd, we nev­er see that in our every­day life, we are told. But con­sid­er, for exam­ple, the vari­ety of dogs on the plan­et… We humans made them… by con­trol­ling which dogs shall mate with which…. In the short peri­od of 8 or 10,000 years, we pro­duce this immense vari­ety of dogs. Now com­pare that with four bil­lion years of bio­log­i­cal evo­lu­tion, not arti­fi­cial selec­tion, but nat­ur­al selec­tion, which goes into not just the over­all per­son­al­i­ty and char­ac­ter­is­tics of the dog, but the bio­chem­istry and inter­nal organs… and then it is clear that the beau­ty and diver­si­ty of life on earth can emerge. But if you don’t buy four bil­lion years, you don’t buy evo­lu­tion.

Sagan fre­quent­ly cit­ed this fig­ure of 4 bil­lion years for the ori­gin of life on Earth. Dur­ing his huge­ly pop­u­lar pro­gram Cos­mos, for exam­ple, he used the num­ber in an accel­er­at­ed evo­lu­tion­ary his­to­ry, which you can hear him nar­rate accom­pa­nied by a nifty ani­ma­tion in the video below. Most sci­en­tists have used that fig­ure or a few mil­lion years ear­li­er. For some time, the actu­al num­ber was thought to be between 3.6 and 3.8 bil­lion years. Recent­ly, as Tim Marcin reports at the Inter­na­tion­al Busi­ness Times, some sci­en­tists have con­clud­ed that “liv­ing organ­isms may have exist­ed on Earth as long as 4.1 bil­lion years ago.”

Marcin quotes UCLA pro­fes­sor of geo­chem­istry Mark Har­ri­son, who spec­u­lates, “life on Earth may have start­ed almost instan­ta­neous­ly” (rel­a­tive­ly speak­ing) after the planet’s for­ma­tion some 4.6 bil­lion years ago. These esti­mates come from car­bon dat­ing, not fos­sils, but just yes­ter­day, Sarah Kaplan writes at The Wash­ing­ton Post, dis­cov­er­ies of “tiny, tubu­lar struc­tures uncov­ered in ancient Cana­di­an rocks” may be evi­dence of ancient microbes thought to be 3.77 bil­lion years old, “mak­ing them the old­est fos­sils ever found.”

Like all new sci­en­tif­ic dis­cov­er­ies, these recent find­ings have been con­test­ed by oth­er sci­en­tists in these fields. And like some dis­cov­er­ies, their ques­tions may nev­er be resolved in our life­times. Sci­ence depends on meth­ods of data col­lec­tion, eval­u­a­tion and inter­pre­ta­tion of evi­dence, peer review, and many oth­er process­es sub­ject to human error. Sci­en­tists must often revise their con­clu­sions and recon­sid­er the­o­ries. No sci­en­tif­ic expla­na­tion is con­clu­sive­ly defin­i­tive in all its par­tic­u­lars. Nonethe­less, Sagan believed that only through the sci­en­tif­ic method could we obtain knowl­edge about the cos­mos and the ori­gin of life on earth that was in any way reli­able. He admired reli­gious ethics and the space reli­gions held for the big ques­tions. Sagan even declared in his 1985 Gif­ford Lec­tures (pub­lished posthu­mous­ly as The Vari­eties of Sci­en­tif­ic Expe­ri­ence) that “the objec­tives of reli­gion and sci­ence… are iden­ti­cal or very near­ly so.” But he did not think reli­gions could answer the ques­tions they asked.

Relat­ed Con­tent:

Carl Sagan Presents a Mini-Course on Earth, Mars & What’s Beyond Our Solar Sys­tem: For Kids and Adults (1977)

Carl Sagan Explains Evo­lu­tion in an Eight-Minute Ani­ma­tion

Watch Episode #2 of Neil deGrasse Tyson’s Cos­mos: Explains the Real­i­ty of Evo­lu­tion (US View­ers)

Carl Sagan, Stephen Hawk­ing & Arthur C. Clarke Dis­cuss God, the Uni­verse, and Every­thing Else

Josh Jones is a writer and musi­cian based in Durham, NC. Fol­low him at @jdmagness