Science Focus ( Issue 21)

9 that the genus Homo migrated out of Africa in a few waves [12]. Modern humans were speculated to have encountered and interbred with the Neanderthals and Denisovans who had left Africa much earlier, when we first spread out of sub-Saharan Africa [12]. The interbreeding hypothesis is now supported by the results of many genetic analyses [13, 14], including the 2% inheritance of Neanderthal DNA exclusively found in non-African genomes in the study above [9]. Not only did we find evidence that reconstructed our genealogy, we al so found t rai t s l i nked to these remnants in our DNA. Published in Nature in September 2020, a genomic segment inher ited from Neanderthals on the thi rd chromosome has been identified as a risk locus for respiratory failure after infection of SARS-CoV-2 [15], meaning that the genomic location may contain a version of gene (scientifically termed an allele) associated with an increased risk of severe infection and hospitalization. That allele concerns around 50% and 16% of south Asia and European populations respectively [15]. Scientists posited that the allele could once confer a significant survival advantage on individuals by its ability to elicit a protective immune response against ancient pathogens, so that it was positively selected for in some populations during the course of natural selection [16]. However, the ancient genes may be unfavorable today as the immune response induced could be overly aggressive and potentially fatal in a COVID-19 infection [15]. Our Second Distant Relative – Denisovans The tree of evolution is continuously being tangled with additional discoveries of human ancestry. Besides the archaic humans Neanderthals, Denisovans lived about 40,000 to 400,000 years ago in Europe and Western Asia [17]. This group of extinct human species was first identified by Prof. Pääbo from the genetic mater ial recovered from a f inger bone fragment collected from Denisova Cave in the Altai Mountains of Russia, which gave rise to the name “Denisovans” [18]. Anatomically, Denisovans were speculated to have an elongated face, a wide pelvis, an increased dental arch and lateral cranial expansion [19]. Genet i ca l l y, t he re was a l so ev i dence of interbreeding between Denisovans, Neanderthals and ancestors of modern humans [11, 20]. Traces of Denisovans can primarily be found nowadays in the genomes of Southeast Asian and Pacific Islander populations, whereas people in other parts of the world contain only a very low or an undetectable amount of Denisovan DNA sequence [17]. Similar to the genetic variations inherited from Neanderthals, genetic variants from Denisovans may also contribute to many of our traits, for example, hair texture, height, sensitivity of smell and immune responses [17]. The Quest to Answer the Unanswered The rise of human ancient DNA research not only feeds our mere curiosity, but also provides us new insights on how the course of evolution shapes human into the creature we are today; or in other words, what makes humans human. The fascinating findings on human adaptation and disease susceptibi l ity through time also reveal many intriguing facts about ourselves, not least, to be aware that there is so much we do not know. How many of you would like to follow the footsteps of Prof. Pääbo? 來自尼安德河谷的人類 有著與別不同的認知模式， 我們人類擁有大多數生物皆無 法比擬的智慧，在科學上被名 為 Homo sapiens ， 拉 丁 語 上解作「智人」。然而，過去 曾有物種與我們有著意外地 相似的外貌，他們也懂得運用 自製工具，例如使用打磨過 的鋒利長矛打獵 [1]。被形容 為我們的「姊妹物種」或「遠親」， 他們的主要遺骸於 1856 年在德國的 尼安德河谷（Neander Valley）首次被 發現，人類學家因此將他們命名為 Homo neanderthalensis ，意思是「來自尼安德河谷 的人類」[2]。 古遺傳學先鋒 — Svante Pääbo 古遺傳學是透過提取 和分析殘留在古代生物中 的遺傳物質來研究過去的 一門學科。在最近四十年， Svante Pääbo 教授徹底 改變了我們對人類進化歷