第 348 期·中文精讀

娜塔莉·卡布羅爾:尋找外星生命

Nathalie Cabrol: Search for Alien Life

行星科學家娜塔莉·卡布羅爾談極端環境中的生命、火星探測與搜尋外星生命的可行路徑。

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中文精讀由 AI 基於原始訪談改寫,非官方翻譯,可能存在偏差或錯誤;表述以原音訊與官方文稿為準。說明

娜塔莉·卡布羅爾(Nathalie Cabrol)在海拔六千米以上的火山湖裡做過自由潛水,創下過高海拔自由潛水的女子世界紀錄。她穿著潛水服,下潛到安第斯山脈那些空氣稀薄得讓人頭暈的湖裡。湖底的微生物群落可能是火星上曾經存在過的東西在地球上的映象,這是她一次次往下跳的理由。

她是 SETI 研究所卡爾·薩根生命宇宙研究中心(Carl Sagan Center for the Study of Life in the Universe)的主任。SETI 這個詞,絕大多數人第一反應是「找外星人」,是射電望遠鏡、是科幻電影裡那種訊號接收器。卡布羅爾做的事情,一半時間在分析火星軌道器傳回來的資料,另一半時間在高原上揹著氣瓶往湖裡跳。

Lex 問她,你這份工作要是發招聘廣告,是不是得寫成一百多年前沙克爾頓招募南極探險隊員那種文案:危險旅程,微薄薪水,刺骨嚴寒,數月極夜,九死一生,有意者從速報名。

她說根本就沒有這麼一份招聘廣告。

問題決定你去哪兒,不是反過來

科學家的路徑常常是這樣走出來的:腦子裡先長出一個問題,然後這個問題逼著你去找一個能回答它的地方,最後那個地方可能在地球上最不講理的一片荒原裡。

卡布羅爾讀博的時候研究火星上的水,具體看的是河谷、是水的歷史演化。到了博士後階段,她開始盯另一件事:那些水後來積在哪兒了。當時整個領域的注意力都在河道、在災難性的洪水沖刷、在滲流,沒多少人認真討論「積水的湖」和「積水的海」。她回憶那場學術爭論時說得很輕描淡寫:大家在這種問題上都有點猶豫,來回拉扯了一陣。

她和她丈夫一起提出,火星上的撞擊坑裡可能有過湖。為什麼非得是撞擊坑?因為當時能用的資料只有維京號(Viking)的,解析度和地形測量精度差到沒法在地表上找出一片真正低窪的區域。

唯一確定的資訊是,地上有一個坑,那就是撞擊坑。(卡布羅爾)

換句話說,是資料的解析度逼著人做出了一個技術性選擇。這個選擇本身談不上優雅,它是當時唯一的出路。

維京計劃 1976 年登陸火星,兩個著陸器加兩個軌道器,那是人類第一次真正把腳踩在火星表面上,雖然它們不會動,只是坐在那兒。

到了九十年代,卡布羅爾還在用維京的資料。火星全球勘測者(Mars Global Surveyor)還沒上天,她碩士論文和博士論文都靠這批老資料做完,前前後後幹了二十年。她拿到的第一筆啟動經費,四萬美元,是一筆自由支配的小額基金。

這筆錢在當時看來微不足道,但它開啟的是一條完整的思維鏈條:先搞清楚三十五億到四十億年前火星的環境是什麼樣,再在地球上找一個類似的環境,再看看那種環境裡還能活下來什麼生物,最後反推,要什麼樣的儀器、什麼樣的解析度,才可能把那點殘餘的生命訊號從石頭上摳出來。

從「火星人」到「火星微生物」,人類只用了三個月

水手四號(Mariner 4)是第一個成功拍到火星照片的探測器。在它之前,人類的期待被望遠鏡餵養了幾百年。火星有極冠,有季節,自轉軸傾角和地球幾乎一樣,看過去跟地球相似得讓人浮想聯翩。

當時的很多人以為我們會看到水晶城市、看到穹頂,看到另一個太陽系裡與人類平行演化出來的文明。(卡布羅爾)

照片傳回來,火星看起來像月球。失望是巨大的。

然後水手九號(Mariner 9)來了,一切翻盤。中間還插了一段戲劇性的插曲:火星恰好爆發了有記錄以來最大的全球性沙塵暴,軌道器繞著火星飛了三個月,什麼也看不見。等塵埃落定,火山、峽谷、古河道、沙丘場、極冠一個接一個冒出來。

卡布羅爾說了句很有意思的話:我真要跟你描述火星,不需要造任何新詞。有山、有谷、有沙丘、有冰蓋,全是地球上現成的詞彙。火星文明的神話就此破滅,但科學家的想象力反而被點著了,因為眼前這顆星球變得完全可以用已知的地質語言去描述。

維京計劃隨即被推上快車道。當時卡爾·薩根(Carl Sagan)、克萊因(Klein)這些人已經在琢磨怎麼設計實驗去測火星上有沒有生物學活動。維京上面那幾臺實驗就是這麼來的。

結果是一團混亂。到今天還有人堅信維京當年發現了火星生命,因為其中某個實驗給出了一個奇怪的訊號。主流的看法是,那可以用今天已經觀察到的化學反應解釋掉。

資料太混亂了,混亂到 NASA 決定:如果我們真的想認真找火星生命,就必須先搞清楚環境。因為生命和環境是共同演化的。(卡布羅爾)

這句話是整個訪談裡最關鍵的一條方法論。生命和環境之間的關係,像先有場地再放球員那樣簡單。一顆行星提供了物理化學的邊界條件,這些邊界決定了生命能不能出現;一旦生命出現,它會反過來改造這顆行星。

地球歷史上最大的一次「生命改造星球」事件,是 32 億年前大氣裡被注入大量氧氣。這件事改寫了整個地球的表面化學,也改寫了我們從太空中看過去的光譜特徵。

所以找火星生命這件事,變成了一個二元方程組:你知道環境,但不知道生命;你知道生命可能存在,但不知道當時的環境。兩個未知數。維京的失敗,本質上是一次解題順序上的錯誤。

如果火星上真有過生命,它會留下什麼

地球用掉了自己地質歷史的 82%,才從最簡單的微生物走到複雜生命。這裡說的「複雜」,還不是指人類,是指動物。

火星比地球小,磁場丟得很快,大氣也丟得很快。假如生命在火星上真的出現過,那麼當環境開始惡化時,它大概還停留在簡單生命階段。而那個惡化的時間點,距離行星形成還不到十億年。

所以今天的火星表面,幾乎不可能留下任何活的東西。能指望的只剩下形態學和化學的痕跡。

形態學上最好的參照物是疊層石(stromatolites),一種由微生物一層一層堆出來的岩石構造。我們已經知道怎麼識別它們。化學層面上,蓋爾隕石坑(Gale Crater)有個耐人尋味的問號:那裡檢測到碳 12 的富集。碳 12 在地球上通常被生命優先利用,但非生物過程也能造出同樣的訊號。

所以它本身算不上真正的生物標誌物,但它讓人忍不住盯著看。(卡布羅爾)

再加上甲烷。這兩樣東西湊在一起,構成了一個既不能證實也不能證偽的懸案。三十五億年前的火星,表面被撞擊事件反覆撕碎,但確實還有一些極古老的岩石倖存下來了。火星車之所以都往古湖泊和撞擊坑跑,就是因為那裡的石頭夠老,老到有可能儲存下那個視窗期的記錄。

撞擊坑這件事,對生命既是建設也是毀滅。構成生命的那些磚塊,也就是碳、氫、氧、氮、磷,是星球吸積過程中由小行星和彗星一路帶過來的。一顆小行星砸下來,毀滅性驚人,但它同時也把生命的原材料和大量能量一起送到了現場。如果周圍環境合適,說不定就能湊出一鍋湯來。

泛種論不解釋生命的起源,它只負責送貨

這就引出了泛種論(panspermia):彗星和小行星體內本來就帶著生命的建材,只要落在條件合適的行星上,就有可能把它「種」下去。

Lex 追問,你心裡有多大比例相信,地球或火星上的生命其實是別處送來的?

卡布羅爾把這個問題拆開了。

泛種論是一種載體,是一種可能的運輸機制,它把生命的材料撒得到處都是,但它並不解釋生命是怎麼起源的。(卡布羅爾)

這個區分很重要。很多人把「原料到處都是」和「生命一定會出現」混為一談。實際上,構成我們的那些元素在星際介質裡極其普遍,宇宙中到處都是,它們只是在等一個合適的環境。而那個環境需要什麼,地球已經給出了答案:水、能量、庇護所、營養。

Lex 丟擲了一個頗為激進的設想:既然地球文明也許只能再撐幾百年,不如造一門「生命炮」,把地球上的微生物打包射向宇宙,主動製造泛種。他甚至分了兩個層次:一個是把人類意識、複雜資訊複製出去,另一個更簡單粗暴,直接把一堆細菌、一堆地球上最皮實的生物扔出去。

卡布羅爾提醒他,後面這個方案送出去的東西是活生生的生物體,已經越過了「建材」的範疇。這確實是一條捷徑,但它繞過了起源問題,算不上回答了起源問題。

至於把 DNA 存進各地的保險庫、或者存到月球上,這類專案現在已經在做了。有人真的在討論把 DNA 放到月球。再往前一步就是超人類主義:用 AI 和機器學習去增強人類本身。

她的回應帶著一種長輩式的清醒。

備份當然是好事。但對我來說,照顧好這顆行星,回到與環境和 equilibrium 的狀態,也許才是最好的備份。然後讓演化去做它該做的事。(卡布羅爾)

她接著說了一句相當不客氣的話:我們現在像是一群青少年,腦子夠用,能造出很酷的工具,但腦子還不夠用,理解不了自己在做什麼的後果。而賬單現在已經開始寄到家裡了。

放在德雷克方程裡,這就是那個 L,智慧文明能維持可探測狀態的時間長度。文明從雷達上消失的方式有好幾種:自我毀滅、被外部事件抹掉,或者進化到與宇宙完全同頻、徹底融進背景噪聲裡,從此不可分辨。

還有些更野的猜想。有人提出,我們觀測到的暗物質,說不定就是某種生物過程。卡布羅爾沒有為這類猜想背書,只是把它擺出來,作為一種可能性存在。

一百二十三種定義,和一種「不知道生命是什麼」的誠實

訪談進行到這裡,話題從火星轉向了更根本的地方。

關於生命,我們有一百二十三種定義。也有人說,我們其實一個定義都沒有,我們只有對生命的描述。(卡布羅爾)

這句話聽起來像玩笑,實際是這門學科的真實處境。我們要找的東西,連它是什麼都說不清,手上只有幾條關於它性質的線索。

最早的一條線索來自薛定諤,四十年代那本《生命是什麼》。當代最常被提起的是傑里米·英格蘭(Jeremy England)的生物物理論:生命是熱力學的必然結果,是抵抗熵增的最優解。

如果這個說法成立,那意味著生命在地球之外同樣成立,它是物理學在宇宙任何角落都會給出的同一個答案。物理學處處相同,那麼生命就可能處處滲透。

卡布羅爾提出了一個我讀到時停下來想了很久的類比。

她讓讀者去看人類語言的結構。語言千差萬別,但功能完全一致:交換資訊、儲存資訊、理解資訊。一個細胞做的事情,本質上也是這個。

再看語言的層級:先有字母,字母組合成音節,音節組合成詞,詞本身只是靜態的指稱,得有動詞給它方向,然後才是各種補語。

生命的層級是:原子,原子組成無機分子,無機分子變複雜,然後是更復雜的無機分子,再到有機分子,再到 RNA、DNA。

我們的語言結構,和生命的結構是同一套腳手架。我們只是在重複同一種模式。我管這個叫曼德博宇宙、分形宇宙,因為它本來就是這樣。(卡布羅爾)

字母對應原子,音節對應簡單分子,詞對應有機分子,動詞對應 RNA 和 DNA,補語對應蛋白質和各種調控層。這個類比在嚴格科學意義上當然算不上證明,但它提供了一個視角:資訊處理這件事,從一個細胞到一門外語,走的是同一條路。

所以她說,我們該往內看,而不只是往外看。我們本身就是生命的表達。搞清「生命的本質」,比搞清「哪裡有生命」更有價值。因為一旦抓住本質,你就得到了一個通用的生物標誌,它不再依賴對方是否使用和我們一樣的生化反應。

宇宙大機率到處是藍細菌,複雜生命則是另一回事

為什麼找「像我們這樣的生命」這件事算不上偏見?她給了兩條理由。

第一條是元素的代際。構成我們的碳、氫、氧,是上一代恆星死亡時鍛造出來的。我們的太陽屬於最年輕的一代恆星。第一代和第二代恆星基本只有氫和氦,或者金屬極其貧乏,它們造不出這些東西。它們死掉,才有了太陽這樣的恆星,才有了我們這套化學。

第二條是分佈。開普勒(Kepler)、TESS,以及現在的詹姆斯·韋布(James Webb),已經在告訴我們,宜居帶裡的行星數量驚人,型別也極其多樣。像太陽這樣的恆星數不清,但更有意思的是別的型別恆星:它們也有宜居帶,而壽命最長的那類,可以比太陽長一千倍。

一千倍是什麼概念?太陽的壽命大約一百億年。在這類恆星周圍,一顆行星可以穩定地宜居上萬億年。你可以在那種地方盡情想象會發生什麼。

而簡單生命出現得極快。地球上的生命幾乎是一有條件就冒出來了。因此她的判斷是:

宇宙裡大概到處是藍細菌。但複雜生命,那是另一回事,需要更多時間。(卡布羅爾)

複雜生命躍遷的扳機在哪裡,至今沒有答案。寒武紀大爆發把地球推向了今天的我們,但中間插了一槓子恐龍,恐龍又被小行星抹掉。如果那顆小行星沒來,演化的走向會完全不同。

她提到幾層變數,層層疊加:

  • 星系位置。我們在銀河系的宜居帶裡。太靠近中心,恆星密度太高,太陽系外圍的奧爾特雲(Oort Cloud)會被引力擾動撕開,彗星被大量拋向內太陽系,撞擊頻率飆升。再加上輻射,銀河系中心是個不折不扣的危險街區。
  • 氣候歷史。有一個還在等觀測證實的假說:演化的大跳躍似乎與「雪球地球」事件相關聯。直覺上你會認為全球冰封應該導致生命萎縮,但每次雪球事件之後,反而出現了演化上的大躍進。為什麼,不知道。
  • 大滅絕。百分之九十的物種被抹掉的地質事件,會重新洗牌演化的方向。

所以,一顆比地球宜居得多的行星,會不會走出和地球一樣的路?她的回答是不確定。取決於滅絕事件,取決於氣候,取決於一堆我們還列不全的變數。

但有一點她說得很確定:在尋找生命起源這件事上,我們有偏見,而且這種偏見是合理的。因為地球是唯一的模型,而這個模型背後有數字支撐,這就是中庸原則(principle of mediocrity)。地球極有可能代表著一大類行星。

當然,別的生物化學也存在。土衛六(Titan)就是候選:那裡有碳氫化合物的湖、有雨、有一整套完全不同的溶劑體系。我們正在往那兒去,去了就會學到東西。

而尋找「生命的本質」這件事,沒有這種偏見。因為物理學在宇宙各處都一樣。

「宇宙正在透過我們變得有生命」

卡布羅爾提到量子物理裡那種非局域的關聯,光子與光子、原子與原子。她說自己可能是在錯誤地把兩件事拼在一起,但在她的圖景裡,宇宙處處相連。

Lex 順著這個往下推:那生命的連線也是一種滲透整個宇宙的東西。找到別處的生命起源,只是讓「生命的本質」在我們眼前顯形。

我認為我們最大的成就,會是找到那個過程本身。到那一刻,宇宙會用它自己的生命力把自己點亮。我只能管它叫一種活的力量。(卡布羅爾)

她描述的是一個隨時間變得越來越複雜、越來越能收集資訊、並且最終回過頭來理解自己的宇宙。薩根那句話在這裡落地了:我們是宇宙理解自己的一種方式。

然後 Lex 說出了他心裡那點不太舒服的東西。他看著 AI 在自然科學裡的突破,蛋白質摺疊、物理模擬,忍不住想:最終解開生命本質之謎的,可能不是人,是 AI 系統。這讓他有點難過。

卡布羅爾的回答很乾脆:她還不買賬。

AI 沒有意識。AI 是人造的,所以它是工具,一個極其聰明的工具。只要我們造它、用它當工具,它就還是工具。(卡布羅爾)

她說這行當裡關於奇點的喧囂,有大量是科幻電影添的柴。

Lex 正面頂了回去。他舉 AlphaZero 對 Stockfish 的例子:AI 確實比最強的人類棋手更強,而且強的方式是人看不懂的。它會犧牲一個兵、一個後、一個象,人類棋手先是直覺地感到「這可能有道理」,然後發現自己其實並沒有真正理解。他說,你可以堅持 AI 沒有意識、不像人那樣「懂」,但那套程式裡確實存在一種人類不具備的智慧和知識縱深。AlphaFold 在蛋白質摺疊上也是如此,把 AI 用到核反應模擬上也是如此。人類用詩意的直覺去觸碰生命的本質,但真正的理解可能來自一個計算能力遠超我們的系統。

卡布羅爾的反駁換了個角度:那套工具本身是人類給自己造出來的能力延伸。

如果這個工具能給我一個新的視角,那它就是工具。我們正在跳進對宇宙更深一層的認知裡,不管是透過我們的大腦,還是透過另一種收集資訊的方式。(卡布羅爾)

她說,人類一直就是這麼幹的:造工具,然後把工具整合進思維方式。下一代人從小泡在這些工具裡,會把它們當成理所當然。現在很多人的心智已經和網際網路長在一起了,記憶方式被重新配置,不再需要背誦大量事實,因為隨時可以查。資源被從「死記硬背」重新分配到「思考」上。

她說的奇點,是人類與技術的共同演化

她在一月要出一本書,先出法文版。英文稿其實早就寫好了,是先從英文寫的,再自己譯成法文,現在在找美國的出版社。

書裡有個核心觀點:人類正在與技術共同演化,與環境的共同演化正在被替換掉。

這在她看來才是真正的奇點。

緊接著她給了一個診斷,這個診斷值得單獨拎出來:我們為什麼把環境搞砸了?為什麼面對大流行病反應如此糟糕?因為我們已經和環境脫節了。

過去,你獲取資訊的渠道就是這顆行星本身。環境給你什麼訊號,你就適應什麼訊號。現在,資訊來自網路、來自手機,而且沒有任何過濾層。

你沒有辦法判斷這些資訊是不是真的。(卡布羅爾)

Lex 在這裡又一次打斷,說他要反駁。文字稿在這裡斷了。但前面這一段已經足夠清楚:一個研究生命起源的人,最後把矛頭指向了資訊生態。

這跟跑題沾不上邊。她的整條思路是連貫的:生命是什麼?是收集資訊、交換資訊、儲存資訊的能力。那麼當一個物種的資訊來源被切斷、被汙染、被從環境裡剝離出來,它在生命演化的座標上處在什麼位置?

她前面說過,生命想要儘可能多地獲取周圍環境的資訊,而複雜性就是獲取、交換、儲存最多資訊的能力。用這把尺子量自己,結論不太好看。

能帶走的東西

這場對話橫跨火星地質、生命定義、星系宜居帶和資訊生態,散得很開,但底層有一條線:搞清楚「生命是什麼」,比搞清楚「哪裡有生命」更根本。前者一旦有答案,後者自動變成工程問題。

對讀者來說,能落地的東西有這麼幾條:

  • 用問題的形狀去找地方,別用手邊的地方去找問題。卡布羅爾研究撞擊坑裡的湖,起因是當時的資料解析度只允許她看撞擊坑。約束條件不掩蓋問題,它塑造問題。
  • 看到「原料到處都是」時,別急著推「生命一定會有」。碳氫氧氮磷在星際介質裡極為普遍,但普遍的是建材,建築還差得遠。中間隔著一整套還沒搞清的環境條件。
  • 關注「生命的本質」這類基礎問題,它們的回報週期長,但一旦突破就是通用解。一個不依賴地球生化體系的生物標誌,價值遠超在任何單一行星上的一次確認。
  • 把資訊攝入當成一件需要過濾的工程來對待。卡布羅爾說人類和環境脫節、資訊沒有過濾層,這話對個人同樣成立。你每天接收的資訊裡,有多少是這顆行星直接給你的,有多少是演算法喂的,這個比例值得定期盤一次。
  • 對 AI 保持工具視角,同時承認它在某些領域已經產生了人類無法直接理解的知識。這兩件事可以同時成立。你可以既不同意奇點敘事,也承認 AlphaZero 在棋盤上看到了人類看不見的東西。

最後留一個她自己給出的判斷,值得反覆咀嚼:我們現在像一群握著先進工具的青少年。工具本身夠好,問題出在成熟度上。而成熟度這件事,沒有捷徑,只能靠把後果一個個吃下去,慢慢長出來。

文稿由本站用語音識別模型(基於官方釋出音訊)機器轉寫,可能有識別錯誤,人名與數字請以音訊為準。
Lex Fridman Podcast #348 – Nathalie Cabrol: Search for Alien Life Machine transcription by Qwen3-ASR 0.6B (MLX, local, Apache-2.0). May contain recognition errors. The following is a conversation with Natalie Cabral, an astrophysicist and scientist at the SETI Institute, directing the Carl Sagan Center for the Study of Life in the Universe. She explores some of the harshest places on Earth, including free diving in volcanic lakes, all in the pursuit of understanding living organisms beyond Earth. For this, she holds the woman's world record for diving at altitude both scuba and free diving. She's amazing. And now a quick few second mention of its sponsor. Check them out in the description. It's the best way to support this podcast. We got True Classic T's for T-shirts, Shopify for e commerce, BetterHelp for mental health and Athletic Greens for the best daily multivitamin. Choose wisely, my friends. And now onto the full ad reads. Never any ads in the middle. I hate those. I try to make these interesting, but if you skip them, if you must, please still check out the sponsors. I enjoy their stuff. Maybe you will too. This show is brought to you by True Classic T's, high quality salt slim fitted T-shirts for men. I happen to be wearing one now. In fact, I'm wearing one every single day unless I'm wearing a suit or dress shirt. That's the go to comfortable programming coding outfit. If I want to do work for prolonged periods of time, that's what I'm wearing. Is there anything better than a great black T-shirt? I'm not sure if there is. And now I'm reading through the T-shirt Wikipedia, which again is amazing that there's a Wikipedia article on all the basic objects and things and concepts in this world. Now, so yeah, there's an interesting history based in undergarments in the 19th century, but now there's a big picture of Marlon Brando wearing a T-shirt in the movie of Streetcar Named Desire. I guess, I guess there is a phase shift at some point when people started wearing them. By the Great Depression, the T-shirt was often the default garment to be worn when doing farm or ranch chore. Of course, it's the cowboy thing. So first you start at the farm and the ranch, and then that takes over because that's where all the cool people are, and they define the the fashion of like the rugged American individual. And then it looks like in 1960s printed T-shirts gained popularity for self expression as well as for advertisement protest and souvenirs. Of course, in the 60s, that's when people started getting creative. It's not enough to have a plain T-shirt like I have on now. Anyway, go to trueclassic dot com and enter code Lex to get 25% off. This show is also brought to you by Shopify, a platform designed for anyone to sell anywhere with a great looking online store that brings your ideas to life and tools to manage day to day operation. And I will likely be using it, or I should really use it when I finally get my lazy butt to start doing merch. It's something that a bunch of people ask for, and and I do myself enjoy wearing T-shirts for things that I'm a fan of. And then when people notice, it's kind of like a nod of exceptions to each other. I was just wearing a Metallica T-shirt like yesterday, which one was it? I think it was Master of Puppets. And I used to have that T-shirt many years ago, and it got so worn out because I would wear it everywhere and it just got worn out. And I just saw, I was somewhere I think I was in Boston. It was the souvenir shop, and I saw that same shirt, but it was all crisp and nice. And I I had to have it, but obviously Metallica is a giant band. But I think this applies to even small things like podcasts and so on. I'm just a huge fan of podcasts, a huge fan of a lot of things like audiobooks. I wish there was more merch for individual audiobooks. Maybe that's what I'll do. I'll do like merch for books I love. You know, like books on tape, but it'll be books on shirt. Anyway, lots of ideas. If you have ideas and you want to launch a store, you can go to Shopify, get a free trial and full access to Shopify's entire suite of features when you sign up at Shopify dot com slash Lex. That's all lower case. This episode is also brought to you by Athletic Greens and it's AG1 drink a drink that I just drank. What is it? Ten minutes ago, I drink it usually twice a day, once to break a fast and second time after a hard workout or a long run. It's an all in one daily drink to support better health and peak performance. It has almost become meme. How much I love Athletic Greens. It just makes me happy. I love the taste of it. I like the psychological feeling of knowing I got my stuff together because I'm at least I'm taking care of that when everything else is falling apart when my life is giant mess. At least I got some of the basic nutrients and vitamins into my body. And the rest, of course, I take care of sleep and exercise and all that. But at least this this one thing this is this is a place I go to for reassurance for stability. They'll give you a one month supply of fish oil when you sign up at Athletic Greens dot com slash Lex. This is the Lex Friedman podcast to support it. Please check out our sponsors in the description. And now dear friends, here's Natalie Cabral. You are the director of the Carl Sagan Center for Research at the SETI Institute. SETI of course stands for Search for Extraterrestrial Intelligence. One of the things you do as part of that is travel to some of Earth's most extreme and dangerous environments in search of organisms that live in conditions analogous to those on Mars. First, let me ask what the job posting for the work you do looks like. Is it like Shackleton's ad in 1900 that said people wanted for hazardous journey to the south pole, small wages, bitter cold, long months of complete darkness, constant danger, and also where do I apply? That's funny because there was not really a job application. In fact, when you're a scientist, you have questions in your mind and you have hypotheses and you start to list the kind of thing you need to answer. And then when you you see the kind of thing you have to answer, then you kind of know the places where you need to go to do that. As far as science is concerned, started with analyzing data from Mars missions, and I had written a PhD about water on Mars, first looking at channels and the history evolution of of water. But then during my postdoc, I started to look where that water was pounding. Interestingly enough, everybody was about channels and water and whether catastrophic or whatnot or seepage. But when you are talking about pounding water like lakes or ocean, people were starting wading their arms a little bit. So it was a little bit of a battle. Interestingly enough, yeah, but that got us on track with my husband. We were working together and we started developing the idea the concept of lakes in impact craters. So why in impact craters just because the Viking mission at the time, which is what we were working with, the resolution and the topography were so poor that there was really no way of telling where you had a real low in the topography. The only thing you knew was a hole in the ground was an impact crater. So when you saw valleys. What was the Viking mission? The Viking mission landed on Mars in 1976, and there were two landers and two orbiters. So they were really our first feet on the ground on Mars, but they were lander they were not moving they were not going. And that was the data you were looking at. It was already in the 90s, but we didn't have yet the Mars Global Surveyor and whatnot. We still worked for 20 years. We worked on that. I did my master and my PhD thesis on Viking missions. You mentioned that the places you go to are defined by the questions you want to ask. Let's just step back. What questions have always tugged at your heart? Well, that's the thing. That's why I was looking at those images and saw some lakes, and then came time where we started talking about sending landers and rovers on Mars and looking maybe at the possibility that Mars was habitable. And lakes are particularly good places to look for those questions. So this is how it all ties up. So you were always curious about life out there. I have been always curious about life in the universe and about questions on how we got to be here and and the bigger question now with 25 years more in you know in that business. It's more about understanding the origin and nature of life rather than whether there is life or not on Mars. I mean, this was really for me a stepping stone to bigger questions, but they were definitely important because they helped me frame. My way of thinking about those questions and so looking at Mars at lakes understanding what the conditions were 3.5 billion years ago or close to 4 billion years ago, then I knew the type of environment I needed to explore here on Earth as analogous to be able to understand what type of life. Still survives in those environment and what kind of instrument and what kind of resolution do I need to actually detect it. So this is how the whole thing started and it started with a small grant, literally 40k. It was a discretionary fund and this is how I got started in my in my career. And so many of these questions you can answer by looking at life in extreme conditions here on Earth. But let's let's step back a little bit and look at Mars and lakes on Mars. Just going back to your PhD and before and maybe today, what do we understand about life on Mars? What do we understand about lakes on Mars? Is there water on Mars? What do we understand about the conditions from 4 billion years ago on Mars? Well, we've gone a long way. Remember from from the Viking where we had no resolution. Well, we had a little bit more resolution with than with Mariner. What did you think at that time? Sorry to interrupt. It was just take us back to that mindset. It was really the exploration like your first look at a planet. You have to remember that the first mission that successfully snapped some pictures of Mars was Mariner 4. And then everybody at that time was still under the spell of you know edge wells and and the idea that Mars looked with telescope so similar to the Earth. Polar caps we could see them with a telescope and we knew it had season. The actual tilt is pretty much the same as the one for Earth. So when Mariner 4 left, everybody not everybody but a lot of people thought that we would see the crystal cities and domes and stuff that another civilization might have evolved in parallel to us in the solar system. And of course, when the first images came back and Mars looked with that kind of resolution, it had like the moon. It was a huge disappointment. Then Mariner 9 came, and that changed everything. There there was a little bit of drama because Mars started one of the biggest storm it ever experienced, and so for three months we had an orbiter circling around Mars and not seeing anything. But then when the dust cleared, all of a sudden we started discovering volcanoes, valleys, ancient channels, dune fields, polar caps, and see when I'm talking to you I don't need to invent any words. To describe Mars, and although the myth of extraterrestrial civilization on Mars was gone, all of a sudden. The imagination of the scientists started to pick up because right away we were seeing something that was familiar that we could describe. So right away Viking would put on the fast track and the idea was so Mars looks so much like Earth could have been although it's arid and there is little atmosphere et cetera. Could there be life? And of course behind this at the time there were people like Klein and Sagan Carl Sagan, just you know thinking about how can we test the idea of biology of life on Mars. So this is what Viking did, but of course at the time when the two landers arrived on Mars. We didn't have the context of the geology of the environment. We didn't have much data at all. So the data that Viking sent back was very confusing. Some people still think today that we discovered life on Mars at the time because some of the experiment turned out to show a strange signal. But most of the community think that it can be explained by chemical reaction that we see today. So it was so confusing that NASA decided say okay if we want to be serious about looking for life on Mars, we have to understand the environment because life and environment coevolve. So as cause or effect, a planet is going to give you the physical chemical environment for life to happen. These are the boundaries, but once life is here, it's going to change everything. One of the biggest impact of life was to inject oxygen into the atmosphere of the Earth 3.2 billion years ago, and that changed everything including our signature in space. So there is this coevolution. So if you want to understand one, you have to remove the other from the equation. This kind of a two unknown question. So even though oxygen changes our signature today, what if all life on Earth died and now we fast forward a billion years? What would be the traces left? So the question I'm trying to ask is, if life had existed on Mars, what would be the signs we would look for? That's a very good question. The thing is that if you draw the parallel with Earth, it took 82 percent of Earth history, geological history, to go from very simple life microbial life to complexity. And when I'm saying complexity, I'm not even talking about us. I'm talking about animals. So Mars is smaller, lost its magnetic field very fast and lost its atmosphere is very fast. Life also appeared on Earth very fast. So the condition being quite similar at that time between the Earth and Mars. Let's assume for a moment that life appeared on Mars. It would have been simple life when conditions started to degrade, which was less than a billion year after the planet had formed. So everything at the surface would have disappeared except maybe for morphological traces of the interaction between life and its environment. So on Earth, the best example are what we call stromatolites. These are rock formation that are built by microbes. So we know that we we know how to recognize them. You could have chemical traces as well. There is some interesting question marks right now, but carbon isotopes at Gale Crater because we found an abundance of C12, which normally is used by life on Earth, but it can produce be produced by other things. So it's not that it's a real biosignature in itself, but it's intriguing. We have now the C12 and we have methane. But going back, it's a time on Mars 3.5 billion years ago where you have lots of destructions where you have lots of impact cratering et cetera. So but we still have very old rocks that are that survive from that time. So these are good good places. That's why we are sending the rovers in those places ancient lakes and impact craters and and places where you have very old rocks. So when you say ancient lakes and impact craters, the simple question. So impact crater is a crater created by a giant rock hitting the planet. And yes, a big rock that can be metal or rock or it can be a comet as well mostly ice. So is that good for life or bad for life? For creating life and destroying life. Both it's actually both. Interestingly enough, the building blocks of life—the bricks—the stuff we are made of carbon, hydrogen, oxygen, and nitrogen and phosphorus—they were included. On our planet, they were built in just because our planet is made of this kind of rocks, asteroids and comets coming together by what we call accretion. So they were built in when an asteroid comes. There is a lot of destruction going on, but at the same time, those rocks they bring with them those bricks of life and they create lots of energy. And if the environment around is favorable, you might possibly be have some sitting going on. That's one of the aspect of what we call panspermia, which is the fact that comets and asteroids have the building blocks of life embedded in them, and that given favorable condition, they might be able to seed planets. This is a theory. What percent of you, when you're looking up at the stars and wondering about this stuff, thinks that panspermia has is what happened on Mars or on Earth, which is the building blocks of life came from elsewhere. Well, but you know that's the thing. Panspermia is a vector potential vector, which means that it actually distributes the stuff of life. Left and right, but it doesn't explain the origin of life. It does—it's not the environment itself. It just promotes maybe, and we still have to prove this. But what we know is that the stuff we are made of is very abundant all over the place, including in interstellar medium. So it's all over. It's all over. The idea is that maybe it just waits to have the proper environment, and we know what it needs here on Earth: it needs water, it needs energy, shelter, and nutrient. So you are fundamentally interested in the origin of life and the big leaps that in evolutionary history that could be like an origin of something, origin of eukaryotes, origin of. Photosynthesis, origin of whatever. I just think if we are civilization here on Earth and we survive another few hundred years, I think it would be a good idea to take a big gun and just shoot life out there, like a life gun. Basically, try to create panspermia. That's a good backup solution. So one way is to actually copy our brains and actual humans, some complex information and send it out there. Another way to preserve life is just to like send the basic building blocks, send a bunch of bacteria, a bunch of. Whatever the rugged organisms are on Earth, just send a bunch of those. These are not the building blocks; they are actual organisms. So what? Isn't that a nice shortcut? Or do we want to? Because you said building blocks are everywhere. Yeah, the bricks of life—the carbon, hydrogen, etc. They they were produced by the death of previous stars. So this is how they were produced, and stars like our Sun started to form ten billion years ago. That doesn't mean that the the sun is the only kind of star that produce you know life or evolved life, but actually was produced ten billion years ago. Now what you are talking about is is a little different. Right now there are many many efforts to do the type of thing you are talking about, which is to put our DNA on whatever kind of substrate and preserve it in vaults either in different place on Earth or on the Moon. Some people are already thinking about putting DNA on the Moon. As far as brains concern, and it's drawing towards transhumanism, which is the enhancement of who we are through AI and machine learning. Of course, having backups is a good thing. For me, I would say that taking care of our planet and going back to a place where we are in equilibrium with our environment would be also maybe the best backup possible. And let evolution do its things. Right now we are like teenagers with enough brain to create cool tools, but we don't have enough brain to understand yet the consequences of what we are doing, and right now we are paying for it. So the question is whether we are going to be able to move forward and learn from the mistakes we are making to become a mature civilization. You probably heard of the Drake Equation that would be the L at the very end, the duration. The duration of intelligent civilization. Exactly, and and or at least the length of time a civilization remains detectable. It can disappear from the radar screen literally for a number of reasons. The first one is destroy itself or being destroyed by external events, or it can become so in tune. And with the universe, and so advanced that it disappears because it melts really in the background and it's not visible anymore. There are some wild theories out there saying that a civilization might be so advanced that you cannot distinguish them from physical processes. And that was an example. Doesn't say that this is the case, but some people say imagine that in fact all the dark matter that we we see or we we theorized about is in fact some sort of a biological process. So you can think about a number of things. Personally, I believe that what you talk about about preserving our information is kind of what life does. We we need to look at ourselves as not different of what the little cell that started off was, and this is what tells you not about the origin of life, but in fact the nature of life, which is a lot more interesting to me. Because it is the nature of life is really what is going to give you some universal signature to look for it all over the place and not only around ponds of water for life as we know it, but the nature of life is telling you that life wants to get the most information possible around its surroundings and complexity is in fact the ability to gather and exchange and preserve the most information possible and and so what you are saying is kind of preserving the kind of information we have. So in the things that we are doing. As life happened, and I say happened because we don't know what life is. We have one hundred and twenty-three definitions of life, and some people are saying we don't have any definition. We only have descriptions of life. And and that's true, and that's true. So think about it for two minutes. We are looking for something we don't know it is, but we have a few clues about the nature of life. There are some really good theories. The first one was Schrödinger right in the forties. Right now there is a guy named Jeremy England. It's another biophysical theory of life. It says life is the inevitable result of thermophysics. This is the best way to beat entropy to fight entropy. But when you look at what we are doing, if you want to know what the nature of life is, look at our languages and they can be very different languages, but they they all have the same purpose, right? Exchange information, understand you know store information and and also whether it is with somebody at the outside or thought in yourself. That's the same thing the cell was doing, but now when you are looking at life and at the structure of our languages, life started with an atom. So it's an atom. They got together to create inorganic molecules. Then you have complex inorganic molecules. Then you get to organic molecules, complex inorganic molecules, and then you have RNA, DNA, etc. Look at the structure of our language. We created alphabets, letters. That's your atom. Then we put them together to create syllables, right? Those syllables get together to create words. Words tell you something, but they are nothing without a verb that gives the direction. That's RNA and DNA, and and then you can put all the complements you want. Our languages are built exactly as life is built. We are repeating patterns. I call this the Mandelbrot universe and the fractal universe because this is exactly what it is. I would say that as much as I do believe to sending probes to explore the universe, I say we should also look inward to find the the answer to some of the profound question of who we are, what's life, what's the nature of life because we are expressing life. So searching not for life, but for the nature of life. The nature of life absolutely. I am more interested in that because the day we understand the nature of life, then we have a universal bio signature and it doesn't matter whether this life respond to the same kind of biochemical processes as we do. Although it makes sense, I I told you about the generational aspect of the bricks of life, the stuff we are made of. The sun is part of the youngest generation of stars, and the first two generation of stars didn't produce the kind of elements we are made of. They were stars that were either without metal, just made of helium and hydrogen, or poor in metals. So the stars died off, and stars like the sun were born from those, and this is why we have elements like carbon, hydrogen, oxygen, etc. Now, and and that's the life we are built on. So I I think it's not stupid to be looking for something that looks like us because right now in the universe this is the stuff that's the most abundant and we see with the exoplanet with Kepler with Tess and and and now with James Webb we see that there are many many different type of planets that may be habitable in the habitable zone of their stars. There are countless stars like the sun, but more interestingly enough there are other type of stars where you do have habitable zone as well and where the duration of the stars are sometimes a thousand times more than our sun. So you can imagine all sorts of things, and you can imagine what type of life would be on around those stars. The biochemistry might be quite similar in fact, and especially for the simple life because simple life starts really quickly on Earth. So my take on this is that the universe is full of cyanobacteria, but as far as intelligent life, it takes more time. So that can take different you know aspect. Do you think it's possible that the universe is full of bacteria and even those stars that last a thousand times longer than the sun, even even on the planets that orbit those stars, maybe bacteria for billions and billions and billions of years. We actually don't know what triggered the evolution to complexity on Earth. That's still a big question mark. Is that the most impressive invention on Earth to you? That Cambrian revolution is really what took us towards what we are, and in the meantime there were the dinosaurs, etc. The dinosaurs were wiped out, so the evolution could have taken a completely different turn. It's always I would say mass extinction that are going to drive what's the end game, but. Yeah, you take two planets and you change. You know those asteroid impacts or those big geological events that wipe out like ninety percent of life at any time. The thing that seemed to be interesting, there are two things. The first one is where you are located on our galaxy matters a lot. We actually are in the habitable zone of our galaxy, and if you are too close to the center, then it's a lot denser. And remember, we have the orb cloud around our solar system, and if you are in the region of the galaxy that's too populated, then you are going to run interaction gravitational interaction with all these stars, and since it's more dense, you will have more of the comets that are living in the orb cloud being ejected from the orb cloud and coming towards the inner solar system and collide with planets. So you will have more of these impacts if you are too close to the center of the galaxy. Not to mention the radiation, there is a place in our galaxy where it's a really bad neighborhood. You don't want to be there. You wouldn't be able to have life. But what really matters is extinction, but also the climate history of a planet has a role to play, and it seems that. It's a theory. It still has to be backed up by more observation, but there is a good correlation between not only the passage of the solar system towards the center of the galaxy. There is one place where we get hit by asteroid because of the interaction I was telling you about, but the other one is the climate with the Milankovitch cycles. Big jumps in life evolution seem to be associated with. Snowball Earth episodes. We don't know why yet. Snowball episode. Intuitively, you would think that they are connected to a decrease in life because the whole Earth is covered in ice, but for some reason there were big jumps in evolution right after each of those episodes, and and today there are other things like why all of a sudden you have mutation that seems to be responsible for you know a big jump in evolution. We are not clear yet, so all of those things when you are thinking about life elsewhere are going to come into play. And I cannot tell you that a planet that remains habitable for much longer than the Earth will have an evolutionary path that's the same or different depends on extension depends on climate depends on. What not? It's a little bit surreal. There were two descendants of apes. Well, I think that some some people try to figure out what what the heck is going on, and I mean we're very biased even as so you we're biased as humans, you are less biased as a scientist, but we still love Earth, we still don't know anything but this Earth, and so even though you try to get escape from thinking of what life is. In the search for the nature of life, we still kind of connected to the way we understand the nature of life here on Earth. So I think that it's a little different than that. We are biased when it comes to the origin of life because well we are the only model we know, and as I said it makes sense because it seems that a lot of you know stars like the sun appeared ten billion years ago and there are lots of world that really resemble the Earth and lots of water out there and lots of condition that could be a repeat of what we know and we know that this biochemistry works. So as again as I mentioned, what is going to change is really the evolution of a planet, extinction, geology, etc. But our model is probably very abundant. I am not saying that the end game is going to resemble us because of all this extinction, etc. But this is a good bias. It's one that has the number for it. You know the principle of mediocrity. I think that in that case it really applies where the Earth is representative of an abundance of other worlds. Now, of course, there can be other biochemistry. We have some examples in our own solar system. Titan might be a representative of that. We are not very clear the kind of biochemistry that can come out of a world where you have hydrocarbon lakes and rains and and things like that, but we are going there. So we will learn something about this. So the bias is right there. The nature of life is different. If really life is the best way the universe has to fight entropy, there is no bias there because physics is the same all across the universe. At least the universe we know there might be other universes, but the one we know works with the same physics. So if life is the the best way to fight entropy, you can imagine that life permeates the entire universe, and then the question might change to like flavors of ice cream. What are the flavors of complexity that this process, this nature of life leads to, and there we might have bias about what complexity looks like, what beautiful complexity looks like. We look at humans that operated a certain physical scale and time scale, and we think this is intelligence. We have another problem. We don't know what life is. We don't know what intelligence is, and we don't know what consciousness is. But we are trying to tackle the big question. But do we know what complexity is also? You know no, I think that we have to be honest, and as a scientist, and I am going to step back and talk about intelligence. For me, a bacteria that has survived like cyanobacteria that has survived just like us for billion years in one incarnation or another, and actually they are very similar to the one that they were three point five billion years ago. It has some intelligence about its environment. So for complexity, it might be that we need to take the world literally, which is an assemblage or additional capacity to gather, collect, store information. Maybe this is something like that, or actually use that information to do something with it. But I do completely agree with you when you talk about flavor of ice cream. I think this is exactly it, and I have a basic education about what physics is doing right now, and I am look at quantum physics and what it says about the universe and about the connection about an atom here and an atom here, a photon here and a photon there, and I am starting to put maybe. Wrongfully two into together, but in my mind, and of course it's nothing until I can prove it. But in my mind, the universe is connected everywhere in all different places. So this life connection is something that as you said permeate the universe and the way to find life. Might be very different than to look for the origins of life. I think it's a good thing to go out there and look for the origin of life somewhere else because it's the manifestation of the nature of life that all of a sudden becomes apparent, evident to our eye, but. What I think would be our greatest achievement is that if we can find that process of life. Because at that point in my mind, the universe suddenly is going to illuminate itself with actually its living force—what I can only call a living force. To me, this is what we are looking at: a universe that becomes more and more complex with time, more and more able to gather information, and interestingly enough, why to understand itself. So Sagan was right when he was saying, "We are the universe trying to understand itself, and the more we go, the more the universe become alive, maybe intelligent and maybe also conscious." Conscious, self aware. Exactly. Through us, it does make me a little bit sad as a human, just watching all the breakthroughs on artificial intelligence side when applied to natural sciences, now more and more to physics. That the creatures that will solve the question of the origin of the nature of life or just the process, the nature of life will be AI systems. Makes me a bit sad too. I don't think so. Because you think humans will. At this point in time, remember who was behind AI. You know, I I am not buying in the singularity thing yet. AI is not aware. AI is being built by humans, so AI is a tool and extremely smart tool. As long as we build it and as long as we use it as a tool, it remains a tool. And I think there is a lot of bruh hahah and of course science fiction and movies that don't help. Yeah, I got to push back a little bit. Yes, I agree with you for the most part in terms of bruh hahah and sci-fi. But there is like in the work of Deep Mind, we can look at chess, we can look at protein folding. So chess is a simple one to first look at. What Alpha Zero, which is their game playing engine, was able to discover in a stockfish about chess humbles the best human players. Not just it's better than them. It comes up with ideas that the humans don't understand, and so the AI now is telling you, even though it's programmed by humans, the AI is saying like. Sacrificing a pawn here is a good idea. Sacrificing a queen or a bishop here is a good idea, and then you start to kind of intuit as a human why, but you don't deeply understand, and you can say that AI is not conscious. It doesn't deeply understand in the way humans do, but there is still a wisdom and a depth of knowledge in that chess playing program that humans. Don't have and the same with AlphaFold with protein folding. There is an other applying it to physics to simulating nuclear reactions and so on. It feels like there might be a way to understand the nature of life, that we can kind of intuit poetically as humans, but the the true understanding will come from a system that's much more computational and sophisticated. Again, you know I would push back on my turn because I still think that humans gave themselves ability. To do that by building that tool, so the idea that the tool you know we are getting into the the Cartesian scale and and and the dark forest and all these things. We can see the world this way at this point in time for me. I still see a great tool now. Whether the sci-fi scenario is going to you know happen etc. I still think that we are far away from from this, but if that tool is capable of giving me a new perspective, it's just that. We are starting to jump into a deeper cognition of of what the universe is. Whether it's through our brain or through a different way of gathering information. Remember, this is what we do. Yeah, humans are able to actually build tools and then like integrate them into their way of thinking. Maybe another generation has to be born that is raised with those tools, but we seem to like. Take for granted all the cool technologies you integrate into your way of thinking. A lot of people are growing up now. Their mind is integrated with the internet. You basically reconfigure the way you memorize things. You no longer have to memorize a lot of facts because you can look them up really quickly. Yeah, and so like, so you reallocate a lot of resources for thinking versus memory of just strict facts, that kind of stuff. When we integrate all that. Yeah, and and you know there I would completely agree with you. In fact, I wrote about this again in this new book that's coming out. When is the book coming out? In January, it will be in French actually to start with. You wrote it in French. Actually, I wrote it first in English. Yes, and I translated into French. So the the English version is already pretty much ready to go if we find a publisher in in the US. But anyways, the point being here that I looked at this as a relationship with technology. As a complete change to me, this is the singularity more than anything else, which is the coevolution of human with technology, not anymore with their environment. Why we are messing up the environment right now? Why we don't respond to pandemic the way we should because we are disconnected to the environment. We are taking our information from and we are adapting from right now exactly as you said. We take the information from the web from the phones etc. We have no filter over that information. Before you are out in the environment, the information you get is the one the planet is sending you. Now this information coming from different way, you have no way of knowing if information is correct or not. Yeah, got to push back on that. No, you look at

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