瀝青碎片在被咀嚼的過程中,它的無菌和疏水特性抑制了微生物和化學降解,讓 DNA 被截取並保留在裡頭,提供了古代人類和非人類生物的 DNA,以及古代人類遺傳、表型、健康狀況等等面向的生存訊息,讓現代人能一窺他們的生活片段。
不是每一次的考古發掘都能發現人類遺體,這種時候,被嚼食過的物體就成了研究人員的唯一指標。不過,在過去,考古學家總懷疑不起眼的它們是否真能提供古代 DNA ?一直到最近,研究者們才真正擁有能從「古代口香糖」提取基因組資訊的工具。從這塊蘿拉的口香糖,他們提取了她的 DNA 和微生物群相,並在當地發現製造工具和屠殺動物的痕跡。除此之外,該處並未發現人類遺骸。
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3. Murray, G. G., Soares, A. E., Novak, B. J., Schaefer, N. K., Cahill, J. A., Baker, A. J., … & Gilbert, M. T. P. (2017). Natural selection shaped the rise and fall of passenger pigeon genomic diversity. Science, 358(6365), 951-954.
不過尼安德塔人、丹尼索瓦人等等曾經共存的古人類們,後來全部滅絕了,只剩智人一種獨存。智人從非洲家鄉,前進世界各地的過程,仍有許多不明瞭的地方。這也是古代 DNA 大顯身手的領域,上千個已經發表的古代基因組,讓我們對人類遷徙、混血、發展的歷史,有了更清晰的認識,也回答不少之前不太清楚的疑惑。當然,解決舊問題之後,也開創了新的議題。
不過眾多古文明中,大家最關心的大概還是古埃及。過往試圖取得古埃及 DNA 的嘗試,由於方法上的限制,結果都有些疑問。所幸隨著技術進步,第一批古埃及木乃伊的基因組,終於順利在今年發表。樣本年代介於新王國到羅馬時期,也就是所謂的「古埃及文明」後期;比較基因組發現,當時埃及人與同時期的中東鄰居-黎凡特人,遺傳上能夠視為同一種人。[11]
歷史記載中的病菌是?
古代 DNA 能回答的歷史之謎,還有疾病。西元 1519 年,中美洲的阿茲特克帝國被西班牙征服者擊敗,但是對中美洲居民更致命的打擊,是 1545 與 1576 年發生的 2 次「cocoliztli」,估計造成 700 到 1800 萬人死亡。歷史學家光靠文獻記載,難以確認如此兇殘的「病菌」究竟是誰。最近古代 DNA 研究,指出兇手應該是傷寒(不過該論文尚未正式發表)。[12][13]
也有一些古代 DNA 研究,對象是野生動物,例如之前提過的草原野牛,還有真猛獁象(Mammuthus primigenius)、大西洋鱈等等。如真猛獁象、旅鴿(Ectopistes migratorius)般的滅絕生物,古代 DNA 能讓我們在化石型態以外,以另一種層面認識牠們失落的演化史。[22] [23] [24] [25]
也許沒那麼生物或演化,但是很文化
還有一些古代 DNA 研究,本身或許沒那麼有演化,或是生物的意義,卻比較偏向文化面。今年發表,丹麥古墓中菁英維京戰士的基因組,就是個好例子。DNA 分析清楚指出,擁有全套戰士裝備陪葬的墓主,生理上無疑是位女性;使得女生是否能稱為「戰士」,引發一陣討論熱潮。[26]
分析 DNA 當然只能回答墓主是不是女生,無法解決她是否為戰士的爭議;要解決這類問題,需要的是人類的智慧,而非遺傳物質。不過顯而易見,面對這種爭議性的敏感問題,古代 DNA 能提供比過往方法,更可靠的訊息。
這個幾百年前,跨越大西洋的三角形貿易路線,稱作「中央通道(Middle Passage)」,其中一項主要商品,是非洲智人。圖/取自 wiki
從帕波試圖取得木乃伊 DNA 到今天,已經過了 30 多年,世界和那時大大不同,研究古代 DNA 從異想天開的不務正業,變成尖端科學的熱門領域。最近幾年古代 DNA 的研究實在太多,本文篇幅有限,只能大致觸及幾點,卻仍有許多疏漏。
仍有太多有趣的問題等待回答,而古代 DNA 給了我們機會。比方說,台灣不少人對南島語族的歷史感到興趣,然而事實上不只南島語族,住在南亞、東南亞、東亞的漢藏、南亞、侗台等語族,至今仍沒有任何有關的古代基因組研究。這些人的歷史,也都很值得探討。還有一些問題,本來想都想像不到,例如我們的健康,竟然受到幾萬年前遠古混血的影響?
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