人屬起源可能只是場偶然發生的事件

原文網址：https://gwtoday.gwu.edu/origin-human-genus-may-have-occurred-chance

人屬起源可能只是場偶然發生的事件

Kristen Mitchell

一則經常被引用的論述認為比祖先更加聰明且更善於使用工具的人類是因為氣候變遷而誕生。但由喬治華盛頓大學人類古生物學高等研究中心的研究人員發表的新論文卻對此提出質疑。

包含了我們自身所屬的人屬在內，非洲化石紀錄中在280萬到250萬年前，被喻為「雨後春筍」的大量新種動物湧現事件是許多科學家爭論的議題。專家深信像是全球氣候變遷之類的大規模事件才有辦法讓這麼多不同的新物種同時出現。然而，人類學的訪問助理教授 W. Andrew Barr發表的研究卻表示這波新物種的誕生可能只是碰巧發生，或許和氣候變遷沒有直接關聯。

廣為接受的理論認為大型環境氣候變遷發生時，有些物種會滅絕而有些則會誕生，這可能造成化石紀錄中出現一群或是一波新物種。然而，對於生物的湧現還沒有一套定義可供參考，因此對於哪些生物的集體出現可以構成有意義的事件，哪些可以用隨機變動來解釋，專家的意見也不一致。

Barr博士利用電腦模型來模擬在沒有任何氣候變遷事件的發生下，隨著時間經過化石紀錄呈現的樣貌。他發現模擬中物種集體形成的規模跟化石紀錄中觀察到的規模很相似。據他表示，這代表科學家可能低估了隨機模式在物種形成的變動中扮演的腳色。

Barr博士的發現意謂科學家可能需要重新思考，對於人類祖先為何變得更加聰明且進步所提出的廣為接受說法。

Barr博士表示：「包括人屬起源在內，250萬年前發生的急遽物種替換跟氣候變遷有直接關係的說法，在古人類學中有相當深遠的歷史。我的研究顯示種化速率中的隨機波動可以形成同此規模的事件。這項研究帶來的啟發之一是，我們或許需要更加深入研究人屬為何會在該時間地點崛起。」

他將此模式比喻成丟硬幣。如果丟擲硬幣100次，我們會預期看見50次正面和50次反面。然而，如果只看10次結果的話，我們看到的正反差異可能會大上許多，像是有7次正面而僅有3次反面。他說這種不平衡會隨著時間拉長而被平均掉，但從短期角度仍可以看到許多這類的獨立丟擲硬幣事件。

類似地，Barr博士的模型當中也可以明顯看出物種替換速率的波動，但其僅由隨機過程產生。

Barr博士表示：「認為人屬起源是氣候變遷導致的急遽物種替換事件中一部份的說法，當仔細檢視證據和比較其他可能的解釋之後，會發現它其實沒那麼站得住腳。」

Barr博士說此研究要求科學家仔細檢視他們對於人類演化史所訴說的故事。他認為使人類和我們祖先有所不同的特徵，像是較大的腦袋和使用工具的能力更為精進，可能是源自於許多因素。

「我們現在可以坐在這裡並講述過去的故事都是因為我們今日演化出來的適應能力。」他說，「但其背後的演化動力我們仍尚未知曉。」

這篇研究刊登於7月31日，題名為「Signal or noise? A null model method for evaluating the significance of turnover pulses.」

Origin of Human Genus May Have Occurred by Chance

An often cited claim that humans, who are smarter and more technologically advanced than their ancestors, originated in response to climate change is challenged in a new report by a Center for the Advanced Study of Human Paleobiology researcher at George Washington University.

Many scientists have argued that an influx, described as a “pulse,” of new animal species appear in the African fossil record between 2.8 and 2.5 million years ago, including our own genus Homo. Experts believe it takes a broad-scale event like global climate change to spark the origination of so many diverse new species. However, W. Andrew Barr, a visiting assistant professor of anthropology, published a report that says it’s possible the pulse of new species could have occurred by chance and might not be directly related to climate change.

It is generally accepted that when major environmental changes occur, some species will go extinct and others will originate, which can create a cluster or pulse of new species in the fossil record. However, there is not a set definition of what is considered a pulse, so experts have disagreed about which clusters constitute meaningful events and which can be explained as random fluctuations.

Dr. Barr used computer simulation to model what the fossil record might look like over time in the absence of any climate change and found clusters of species originations that were of similar magnitude to the clusters observed in the fossil record. This means random patterns are likely under-credited for their role in speciation fluctuation, he said.

Dr. Barr’s findings mean scientists may need to rethink widely-accepted ideas about why human ancestors became smarter and more sophisticated.

“The idea that our genus originated more than 2.5 million years ago as part of a turnover pulse in direct response to climate change has a deep history in paleoanthropology,” Dr. Barr said. “My study shows that the magnitude of that pulse could be caused by random fluctuations in speciation rates. One implication is that we may need to broaden our search for why our genus arose at that time and place.”

He compared the pattern to flipping a coin. If you flip a coin 100 times, you would expect to record 50 heads and 50 tails. However, if you are only looking at 10 coin flips, you could see a greater imbalance, instead recording seven heads and only three tails. This would even out over time, but in the short-run, you could see clusters of these independent coin flips, he said.

Similarly, fluctuations in turnover in Dr. Barr’s model are pronounced, but are caused purely by random processes.

“The idea that the origin of Homo is part of a climate-caused turnover pulse doesn’t really bear out when you carefully look at the evidence and compare it against other possible explanations,” Dr. Barr said.

This research challenges scientists to be careful about the stories they tell about the history of human adaption, Dr. Barr said. Traits that make humans different from our ancestors, like larger brains and greater technological sophistication, could have arisen for a variety of reasons, he said.

“We can sit in the present and tell stories of the past that make sense of our modern day adaptations,” he said. “But these could have evolved for reasons we don’t know.”

The report, “Signal or noise? A null model method for evaluating the significance of turnover pulses,” was published July 31.

原始論文：W. Andrew Barr. Signal or noise? A null model method for evaluating the significance of turnover pulses. Paleobiology, 2017; 1 DOI: 10.1017/pab.2017.21

引用自：George Washington University. "Origin of human genus may have occurred by chance: Paper challenges the claim that the genus Homo originated in response to environmental changes."