起源於草原的最早人類

原文網址：http://www.geologypage.com/2017/05/grassy-beginning-earliest-homo.html

起源於草原的最早人類

2013年，亞歷桑納州立大學的研究團隊，在衣索比亞阿瓦什低谷的雷迪-杰拉魯(Ledi-Geraru)，發現了我們自身所屬的人屬(Homo)出現的最早證據。這具帶有牙齒的顎骨定年結果為280萬年前，比先前所知的人屬化石還要早了40萬年。在此發現之後，研究人員的焦點轉往重建這位遠古人類祖先的居住環境，以瞭解他為何會在此時此地出現。

但要如何重建數百萬年以前特定地區的環境，來瞭解我們的老祖宗居住的地方？

古人類學家把動物化石當作時光機的替代品，以重建過往環境的樣貌。如果動物化石指出牠們會啃食樹葉，就像長頸鹿和猴子一樣，那研究人員就知道當時的環境特徵是以高大的樹木為主，且降雨量相當豐沛。如果化石呈現出牠們以草維生，像是許多種類的羚羊一樣，那當時的環境就是開闊且乾燥的草原。

科學家長久以來便認為全球冷化與草原環境的擴張，為人屬的起源打下了基礎。

人類起源研究所的博士後研究員Joshua Robinson表示：「有越來越多的證據暗示了這種可能性。雖然我們已經能將人屬的起源時間更往回推，但目前為止我們仍然缺乏資料可以直接顯示出當時的環境條件。」

在雷迪-杰拉魯顎骨的發現之後，科學家開始著重於進行有關東非上新世至更新世――從350萬年前至100萬年前――的氣候研究，目的是要檢視這些歷時已久的理論。

此篇刊登在期刊《自然―生態學與演化》(Nature Ecology and Evolution)的研究，共同作者包括亞歷桑納州立大學的研究員Joshua Robinson、John Rowan、Christopher Campisano和Kaye Reed，以及南佛羅里達大學的研究員Jonathan Wynn。這是首度有研究對南方古猿(Australopithecus)轉變成人屬時的生態環境，提出全面性的評估。

對東非的人類化石紀錄來說，280萬年前的這段時期特別重要。在雷迪-杰拉魯西方30公里處的哈達爾(Hadar)，便是著名的阿法南方古猿化石「露西」的發現地點。露西在1974年由亞歷桑納州立大學的教授Donald Johanson發現，定年結果為320萬年前。然而，哈達爾的地層層序卻只記錄到295萬年前，因此遺漏了從南方古猿消失至最早的人屬出現之間，這段關鍵過渡時期的紀錄。

利用牙齒化石的穩定同位素，研究人員發現在雷迪-杰拉魯地區，早期人屬確實跟開闊且乾燥的草原環境脫離不了關係。結果顯示在雷迪-杰拉魯地區，幾乎所有跟早期人屬一同發現的動物都是以草維生，包括某些在早於280萬年前還以大量樹葉維生的物種。但是，雷迪-杰拉魯的早期人屬飲食習慣似乎跟較早之前的南方古猿沒有區別，意謂飲食習慣的改變並非是人屬起源的重要特徵。

人類起源研究所的副研究員、人類演化與社會變遷學院的副教授Chris Campisano表示：「我們並不驚訝早期人屬的飲食跟南方古猿相當類似。但我們驚訝的是，當其他所有生活在同一片土地上的動物都改變飲食特性時，早期人屬竟然沒有這樣做。」

將雷迪-杰拉魯地區放入區域觀點後，顯示出當時東非各處的環境變遷並非一致。300萬年前左右南方古猿消失時，阿瓦什低谷為潮濕且以樹木為主的生態系統，到了280萬年前早期人屬出現時，則轉變成乾燥且開闊的草原環境。

人類演化與社會變遷學院的研究生John Rowan表示：「雖然在哈達爾地層序列中，可以看見露西所屬的物種挺過多次環境變遷，但該物種似乎無法度過在上新世晚期，十分開闊的草原環境擴張至阿法地區的事件。」

此外，這些結果也指出雷迪-杰拉魯地區草原環境的擴張，發生時間早於肯亞圖爾卡納盆地和衣索比亞。當時這兩處仍然是以樹木為主，而可以餵養以樹葉維生，以及樹葉和草皆能攝食的哺乳類。

「利用數種不同的棲地指標，我們可以改良之前對各個盆地生態系的重建結果，而能夠辨識出草原擴張過程的細節。」人類演化與社會變遷學院的首席研究員暨主任Kaye Reed表示。她同時也是人類起源研究所的副研究員。「我們正計畫利用同樣的研究方法，來比較東非的其他人類考古遺址。」

Grassy beginning for earliest Homo

In 2013, an ASU research team found the oldest known evidence of our own genus, Homo, at Ledi-Geraru in the lower Awash Valley of Ethiopia. A jawbone with teeth was dated to 2.8 million years ago, about 400,000 years earlier than previously known fossils of Homo. After the discovery, attention turned to reconstructing the environment of this ancient human ancestor to understand why there and why then.

But how do you re-create specific environments from millions of years ago to understand where our ancient ancestors lived?

Paleoanthropologists use animal fossils like proxy time machines to re-create what past environments were like. If animal fossils indicate browsing on tree leaves, like giraffes and monkeys do, then they know that the environment was characterized by woody trees and significant rainfall. If the fossils suggest grazing on grass, as many antelopes do, then the environments would have been open and arid with grassy plains.

Scientists have long suggested that global cooling and the spread of grassy environments set the stage for the beginnings of Homo.

“A growing body of evidence has hinted at this connection,” said Joshua Robinson, postdoctoral researcher with the Institute of Human Origins, “but, until now, we had no direct environmental data for the origins of Homo now that its been pushed back in time.”

Following the discovery of the Ledi-Geraru jaw, an intensive environmental study of the eastern African Plio-Pleistocene — from around 3.5 million years ago to 1.0 million years ago — was conducted in order to investigate these long-standing hypotheses.

The study, coauthored by ASU researchers Joshua Robinson, John Rowan, Christopher Campisano and Kaye Reed with University of South Florida researcher Jonathan Wynn, in the journal Nature Ecology and Evolution, offers the first comprehensive assessment of the ecological contexts of the transition from Australopithecus to Homo.

The time period around 2.8 million years ago is particularly important for the human fossil record of eastern Africa. Thirty kilometers to the west of Ledi-Geraru is Hadar, where the famous “Lucy” fossil of Australopithecus afarensis was found in 1974 by ASU professor Donald Johanson and dated to 3.2 million years ago. The geological sequence at Hadar, however, ends around 2.95 million years ago and is thus missing the important transitional period between the end of Australopithecus and earliest Homo.

Using stable isotopes of fossil teeth, the researchers found that early Homo at Ledi-Geraru was indeed associated with open and arid grassy environments. Results show that almost all animals found with early Homo at Ledi-Geraru fed on grass, including some that consumed substantial amounts of tree leaves prior to 2.8 million years ago. The diet of early Homo at Ledi-Geraru, however, appears to be indistinguishable from that of the earlier Australopithecus, implying that a change in diet is not a characteristic of the origins of Homo.

“We weren’t necessarily surprised that the diet of early Homo was similar to Australopithecus,” said Chris Campisano, research associate with the Institute of Human Origins and associate professor in the School of Human Evolution and Social Change. “But we were surprised that its diet didn’t change when those of all the other animals on the landscape did.”

Placing Ledi-Geraru in a regional context indicates that eastern Africa environments at this time were not Homogeneous. The ecology of the lower Awash Valley shifted from a wet and wooded environment at the time of the disappearance of Australopithecus around three million years ago to a dry and open landscape at the time of early Homo 2.8 million years ago.

“Although Lucy’s species persisted through many environmental changes in the Hadar sequence,” School of Human Evolution and Social Change graduate student John Rowan said, “it seems the species was unable to persist as really open environments spread in the Afar during the late Pliocene.”

Furthermore, these results indicate that the spread of grassy environments at Ledi-Geraru occurred earlier than in the Turkana Basin of Kenya and Ethiopia, which continued to have wooded regions that supported browsers and other mammals that fed on both trees and grasses.

“By using several different habitat proxies, we were able to refine previous ecosystem reconstructions in each basin so that we were able to identify the details of the spread of grasslands,” said Kaye Reed, President’s Professor and director of the School of Human Evolution and Social Change. Reed is also a research associate with the Institute of Human Origins. “We are planning to compare other East African hominin sites using these same methodologies.”

原始論文：Joshua R. Robinson, John Rowan, Christopher J. Campisano, Jonathan G. Wynn, Kaye E. Reed. Late Pliocene environmental change during the transition from Australopithecus to Homo. Nature Ecology & Evolution, 2017; 1: 0159 DOI: 10.1038/s41559-017-0159

引用自：Arizona State University. “Grassy beginning for earliest Homo.” Geology Page, May 15, 2017.