（圖解：2.3萬年前白沙公園地區(qū)的可能樣貌
（發(fā)表于2021年9月23日，作者：英國伯恩茅斯大學(xué)環(huán)境和地理科學(xué)教授馬修·羅伯特·班尼特、英國伯恩茅斯大學(xué)人類古生態(tài)學(xué)首席學(xué)者薩莉·克里斯汀·雷諾茲）

Our species began migrating out of Africa around 100,000 years ago. Aside from Antarctica, the Americas were the last continents humans reached, with the early pioneers crossing the now-submerged Bering land bridge that once connected eastern Siberia to North America.

我們這個物種大約在10萬年前開始從非洲向外遷移。除去南極洲，南北美洲是人類最后抵達(dá)的大陸，早期的拓荒者跨越了現(xiàn)在已經(jīng)被淹沒的白令陸橋，該陸橋曾經(jīng)把西伯利亞東部和北美連成了一片。

At times throughout the Pleistocene ice age, which ended 10,000 years ago, large ice sheets covered much of Europe and North America. The water locked in these ice sheets lowered the sea level, allowing people to walk the bridge from Asia through the Arctic to Alaska. But during the peak of the last glacial cycle, their path south into the Americas was blocked by a continental-wide ice sheet.

在結(jié)束于一萬年前的整個更新世冰期中，巨型冰蓋覆蓋了歐洲和北美的大部分地區(qū)。被鎖在這些冰蓋中的水降低了海平面，這就使人們能利用這座陸橋經(jīng)由亞洲穿過北極地區(qū)走到阿拉斯加。但是在最后一個冰川周期的極盛期，他們向南進(jìn)入南北美洲的通路被一整個大陸那么大的冰蓋封堵住了。

Until now, scientists believed humans only travelled south into the Americas when this ice barrier began to melt – at the earliest, 16,500 years ago. But together with our colleagues, we have discovered a set of fossil footprints that suggest humans first set foot on the continent thousands of years earlier.

科學(xué)家們至今都認(rèn)為：直到這個由冰組成的屏障開始融化時，人類才能南下進(jìn)入南北美洲，最早也是在16500年前了。但我和我的同事們一起發(fā)現(xiàn)了一組腳印化石，表明人類首次踏足這片大陸的時間，比之前認(rèn)為的還要早幾千年。

These footprints, unearthed at White Sands National Park in New Mexico, were made by a group of teenagers, children and the occasional adult, and have been dated to the height of the last glacial maximum, some 23,000 years ago. That makes them potentially the oldest evidence of our species in the Americas.

這些腳印出土于新墨西哥州的白沙國家公園，是當(dāng)年的一群少年、孩童和偶爾混跡其中的成年人留下的，經(jīng)測定，時間是在最后一次冰期的極盛期，大約是在23000年前。這樣，它們就很可能成為我們這個物種涉足美洲的最古老證據(jù)。

Our findings support the idea that humans were present in the southern part of North America before the last glacial peak – a theory that has so far been based on disputed and potentially unreliable evidence.

我們的發(fā)現(xiàn)支持了“人類在最后一次冰期的極盛期之前就在北美洲南部現(xiàn)身了”的觀點(diǎn)，這一理論到目前為止，都只是基于有爭議的以及可能并不可靠的證據(jù)。

Step change

腳步的變化

There are literally tens of thousands of fossil footprints at White Sands. Together, they tell stories of how prehistoric humans interacted with extinct Ice Age megafauna, such as Columbian mammoths and giant ground sloths.

不夸張地說，白沙國家公園里有數(shù)以萬計的化石腳印。它們共同講述了史前人類是如何與已經(jīng)滅絕的冰河時代巨獸（如哥倫布猛犸象和巨型大地懶）互動的故事。
（譯注：哥倫布猛犸象是長毛猛犸象的近親，可能是地球上曾經(jīng)生活過的體型最大的象；大地獺是冰河時期美洲的第三大陸生獸，體重可達(dá)5噸）

The tracks were deposited around the margins of a large wetland – perhaps a lake after the rainy season, but at other times more like a patchwork of water bodies. Until now, the problem had been dating these footprints. We knew they were imprinted before the megafauna became extinct, but not precisely when.

這些足跡沉積在一個大型濕地的邊緣，也許是雨季過后形成的一個湖，但到了其他季節(jié)更像是各個水體匯聚而成的。在今天以前，確定這些腳印的年代一直都是個問題。我們知道它們是巨獸們滅絕前留下的印記，但無從得知具體的時間。

This changed in September 2019 when the team found tracks with undisturbed sediment above and below them. Within that sediment were layers containing hundreds of seeds of the common ditch grass Ruppia cirrhosa. These seeds, when radiocarbon dated, would reveal the age of the footprints themselves. Analysis revealed the seeds range in age from 21,000 to 23,000 years old, suggesting humans made repeated visits to the site over at least two millennia.

情況在2019年9月發(fā)生了變化，當(dāng)時我們的團(tuán)隊發(fā)現(xiàn)，在腳印的上方和下方都有無恙如初的沉積物。在這些沉積物中，有好幾層含有數(shù)百顆很常見的溝渠草（螺旋流蘇）的種子。這些種子通過放射性碳定年法，能揭示出腳印本身的年齡。分析顯示，這些種子的年齡從21000年至23000年不等，這就表明人類在至少兩千年的時間里反復(fù)造訪過這個地點(diǎn)。



（圖解：從這些足印中提取的一部分種子）

The White Sands footprints provide unequivocal evidence that people were in the Americas at the height of the last glacial maximum, rather than some time after, as was previously thought. That’s a big deal for our understanding of the peopling of the Americas and the genetic composition of indigenous Americans.

白沙公園的腳印提供了明確的證據(jù)，證明了和從前認(rèn)為的不同，人類在最后一次冰期的極盛期就已經(jīng)進(jìn)入了美洲，而不是在更晚的時間點(diǎn)。對我們了解美洲的族群分布和美洲原住民的基因成分來說，這可是一件大事。

Using the DNA of modern indigenous Americans, scientists have worked out that their ancestors arrived from Asia in several waves, some of which became genetically isolated. The cause of this isolation is not clear. Now, our new footprint evidence provides an explanation, suggesting that the earliest Americans were isolated south of the North American ice sheet, only to be joined by others when that sheet melted.

利用生活在現(xiàn)代的美洲原住民DNA，科學(xué)家們已經(jīng)得到了答案：他們的祖先分好幾波從亞洲出發(fā)抵達(dá)這里，其中的一部分逐漸成為了基因孤島。發(fā)生這種隔絕的原因尚不明確?，F(xiàn)在，我們掌握的新腳印證據(jù)給出了一個解釋，表明最早的美洲人被隔絕在了北美冰蓋以南，直到冰蓋融化時其他族群的人才加入了他們。

Our discovery may also reopen speculation about other archaeological sites in the Americas. One of them is Chiquihuite Cave in Mexico. Archaeologists recently claimed that evidence from this cave suggests humans occupied the Americas around 30,000 years ago – 7,000 years before people left the White Sands footprints.

我們的發(fā)現(xiàn)也可能重開對其他美洲考古遺址的推測。其中之一就是墨西哥的Chiquihuite洞穴。最近考古學(xué)家們聲稱，該洞穴中的證據(jù)表明，人類大約在3萬年前占領(lǐng)了美洲，這比人們在白沙公園留下的腳印早了7000年。

But the Chiquihuite Cave findings are disputed by some, as stone tools can be difficult to interpret and tool-like stones can form via natural processes. Stone tools can also move between layers of sediment and rock. Fossil footprints can’t. They are fixed on a bedding plane, and so provide more reliable evidence of exactly when humans left them.

但Chiquihuite洞穴（譯注：墨西哥中北部）的一系列發(fā)現(xiàn)遭到了一些人的質(zhì)疑，因?yàn)槟切┦骺赡芎茈y解釋，而且近似于工具的石塊是可以通過自然過程形成的。石器也是可以在各沉積物和巖石層之間移動的。但腳印化石就不能了。它們被固定在了一個層理面上，也便由此成為了更可靠的證據(jù)，證明了人類留下這些腳印的確切時間。
(譯注：層理是巖石沿垂直方向變化所產(chǎn)生的層狀構(gòu)造)



（圖解：左側(cè)為現(xiàn)代人的足跡，右側(cè)為有數(shù)千年歷史的古人足跡）

Teenage kicks
We tend to picture our ancestors engaged in life-or-death struggles – forced to battle the elements simply to survive. Yet the White Sands evidence is suggestive of a playful, relatively relaxed setting, with teenagers and children spending time together in a group.

在我們的設(shè)想中，我們的祖先往往會投入到生死攸關(guān)的斗爭中去，他們被迫與各種因素戰(zhàn)斗，僅僅是為了活下來。然而，白沙公園的證據(jù)表明，那是一個有玩鬧有嬉戲的相對閑適的環(huán)境，少年和孩童們在群體活動中消磨時光。

This is perhaps not that surprising. Children and teenagers are more energetic and playful than adults and therefore leave more traces. Adults tend to be more economical in their movement, leaving fewer tracks.

也許這也不至于讓人驚訝。孩童和少年的精力比成年人更旺盛，也更愛玩鬧，因此留下了更多的足跡。成年人在行動時往往會更簡練，留下的足跡也就更少了。

But another interpretation of this new footprint evidence is that the teenagers were part of the workforce in these early bands of hunter-gatherers. It’s possible that the tracks were left by young people fetching and carrying resources for their prehistoric parents.

但針對這些新足跡證據(jù)的另一種解釋是，在這些早期的狩獵-采集者隊伍中，少年是勞動力的一部分。這些足跡有可能是年輕人為他們的史前父母獲取和搬運(yùn)物資時留下的。

In any case, the people that left their tracks on White Sands were some of the earliest known American teens. Set in stone, their footprints pay tribute to their forebears, who we now know walked the long land bridge into the Americas millennia earlier than what was commonly believed.

不管怎樣，在白沙公園留下足跡的人都是已知最早的一批美洲青少年。他們的腳印亙古不易，在向他們的祖先致敬，我們現(xiàn)在知道了，他們行過長長的陸橋進(jìn)入美洲的時間，比通常認(rèn)為的要早幾千年。

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