尋根問祖 人類祖先之謎變得愈加複雜了

ONE of the oddest things about Homo sapiens is that he is alone. Though storytellers have filled the world with imaginary hominids-from woodland pixies to mountain giants-no sign of the real thing has ever been seen. But that was not true in the past. As recently as 40,000 years ago there were three other species of human on Earth: Neanderthals in Europe, the "hobbits" of Flores, in Indonesia, and a recently discovered and still mysterious group of creatures called the Denisovans, who lived in Central Asia. And now there is evidence that similar diversity existed earlier in human history, a little under 2m years ago, in Africa.

有關智人的最奇怪的事情之一就是他們在人類的進化史上十分孤獨。儘管故事家們想象出了許多人種來豐富這個世界，從森林裏的小精靈到山上的巨人等等，但人們卻從未找到能證明這些人種真實存在的跡象。但從過去來看，這樣的說法並不正確。早在4萬年前，地球上還存在另外三個人種：歐洲的尼安德塔人，印度尼西亞佛洛里斯島上的"霍比特人"，以及最近才發現的一個叫丹尼索瓦人的神祕人種，該人種曾居住在亞洲中部。而現在，人們在非洲又找到了另一個人種存在的證據，該人種在人類進化史上出現的時間更早，距今約不到200萬年。

This evidence, just published in Nature, has been provided by a team led by Meave Leakey of the Turkana Basin Institute in Nairobi, Kenya. Dr Leakey is a member of an illustrious palaeontological clan. Her husband, Richard, discovered in 1967 that the area around Lake Turkana is a good place to look for human fossils and made many important finds there; Richard's parents, Louis and Mary, had earlier been responsible for showing the same was true of Olduvai Gorge in Tanzania; and in 1999 Dr Leakey herself found a new species of hominid, called Kenyanthropus platyops. At 3.5m years old Kenyanthropus predates Homo, the genus to which modern humans belong. Her latest discoveries, however, add to that genus, too.

這些發表在《自然》雜誌上的證據是由肯尼亞首都內羅畢的圖爾卡納盆地研究所的一個研究小組提供的。該研究小組的領導人米芙•利基來自一個著名的古生物學研究家族，1967年，她的丈夫理查德發現圖爾卡納湖附近區域是一塊尋找人類化石的好地方，並在那裏取得了許多重要發現。理查德的父母路易斯和瑪麗曾在早些時候負責坦桑尼亞奧杜瓦伊峽谷的人類化石發掘工作。米芙•利基博士本人曾在1999年發現了一個叫肯尼亞平臉人的新人種，該人種距今已有350萬年，其出現時間比現代人所屬的智人還早。而她最新取得的發現成果又給這一人種增添了新成員。

The problem with studying humanity's fossil record is that it is so sparse: a jawbone here; a braincase there. Often, it is difficult to know if different bones have come from the same species or not. Even multiple examples of the same type of bone can mislead. What looks like two species might actually be the male and the female of one.

研究人類化石記錄最大的問題是資源稀缺，總是這兒一塊顎骨，那兒一塊頭骨。而且研究者通常很難分辨它們是否來自同一人種。即便是同一人種，不同樣本也會給人帶來誤導，兩塊看起來像是來自不同人種的骨頭實際上可能分屬於同一人種的男性和女性。

Such confusion has bedevilled the interpretation of the human fossils found near Lake Turkana. Some palaeontologists see a single, variable species called Homo habilis. Others add a second, Homo rudolfensis. The new fossils found by Dr Leakey and her team (which includes a third Leakey generation in the form of her daughter, Louise) may, however, help clear up what is going on.

這種極易混淆的情況一直困擾着那些想要解讀圖爾卡納湖附近人類化石的研究者們。有一些古生物學家認爲人種是單一的，即只有能人一種，其他人種都是能人的變異體。而另外一些古生物學家則認爲除能人之外還有第二個人種，即盧爾多夫人。而利基博士與她的團隊（該團隊成員還包括利基家族的第三代人，即利基的女兒路易斯）最新發現的人類化石或許可以幫忙理清這些人種之間的關係。

One of the new specimens, known as KMN-ER 62000, has a face like the type specimen of Homo rudolfensis (the fossil that defines the species, if species it turns out to be), though it seems to be from an adolescent, whereas the type specimen is an adult. Crucially, 62000 has a reasonably well-preserved upper jaw, which the type specimen lacks. A computer reconstruction (see diagram) suggests this upper jaw meshes well with the second of Dr Leakey's discoveries, a lower jaw (KMN-ER 60000). She is not suggesting they are from the same individual, since they are of different ages, but they seem to come from the same species, namely Homo rudolfensis.

這些新發現的樣本中有一個被標爲KMN-ER 62000的新樣本，其臉型與盧爾多夫人樣本（該化石是用來鑑別盧爾多夫人人種的一個樣本，如果研究者能用此種方式辨認出樣本屬於盧爾多夫人種）的臉型相似，儘管62000樣本看起來像是來自一個青少年，而盧爾多夫人化石樣本則屬於一個成年人。關鍵是，62000樣本擁有一個保存相當完好的上頜，而這正是這一類型的樣本所缺少的。一張電腦重建圖（見上圖）顯示這一上頜與利基博士發現的第二個人種的一個下頜樣本（KMN-ER 60000）十分吻合。由於兩塊骨頭分屬兩個年齡不同的人，因此利基博士並不認爲這兩塊骨頭是來自同一個人的，但看起來這兩塊骨頭的主人屬於同一人種，即盧爾多夫人。

Equally significantly, a different computer reconstruction shows that the upper jaw of 62000 does not match another famous fossil from the area, known as KMN-ER 1802. This was found in 1973 and had previously been thought likely to be the lower jaw of rudolfensis.

同樣重要的是，另一張電腦重建圖顯示，62000樣本的上頜與KMN-ER 1802樣本並不吻合。KMN-ER 1802這塊有名的樣本化石同樣來自這個地區，1973年人們發現它時曾認爲這塊下頜骨可能屬於盧爾多夫人種。

What 1802 actually is, is now obscure. It is too different from what this chain of reasoning suggests was the shape of Homo rudolfensis to be explained by sexual dimorphism, unless the sexes differed in this species in ways not seen in any other primate. But its previous attribution was made because it does not look much like Homo habilis either.

現在人們仍舊無法弄清1802樣本到底屬於哪個人種。這一推理鏈認爲它是盧爾多夫人，並以人種的兩性差異來解釋其外形差異；但這一差異實在太大，除非這一人種的兩性差異在其他靈長類動物中見所未見方能自圓其說。但之前人們之所以將其歸爲盧爾多夫人種是因爲它看起來也並不像是能人的下頜。

It seems then that at least two, and possibly three species of Homo were stalking the east-African savannah a little under 2m years ago: habilis, rudolfensis and something as yet unnamed. The intertwining history of the human family and the Leakey family has just become even more complicated.

這樣看起，在距今不到200萬年前，曾有三個人種在東非大草原上生活過：能人，盧爾多夫人，以及一個尚未被命名的人種。利基家族一直都致力於研究人類大家族的歷史，現在二者之間的關係交織在一起，使歷史更加複雜了。