早期人類如何進化到吃澱粉 (2/2)

Recently The New York Times on-line reported the following:

How Early Humans Evolved to Eat Starch (2/2)

Two new studies found that ancient human ancestors carried a surprising diversity of genes for amylase, an enzyme that breaks down starch.

By Carl Zimmer

Published Oct. 17, 2024

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Early hunter-gatherers experienced mutations that added even more amylase genes to their DNA. Some experienced mutations that chopped genes out, leaving them with fewer copies than their parents had. But the new studies found no evidence that hunter-gatherers gained any evolutionary advantage from having extra amylase genes.

That changed drastically about 12,000 years ago, the new studies found. It was then, at the end of the last ice age, that a number of societies began domesticating crops, including starch-rich foods like wheat, barley and potatoes.

Across Europe and western Asia, archaeologists have uncovered a vast number of skeletons from this period, from which geneticists have retrieved DNA. The new studies revealed that DNA containing extra amylase genes became more common over the past 12,000 years.

The best explanation for that trend, the scientists concluded, was a pulse of natural selection: People with more amylase genes were much more likely to survive and have children than those with fewer.

Both teams have uncovered additional evidence suggesting that amylase was strongly favored by natural selection not just in Europe and western Asia, but in other places where people shifted to starch-rich diets.

In a study that has not yet been published, Dr. Gokcumen’s group found that extra copies of amylase rose rapidly in the past few thousand years in Peru, where potatoes were domesticated over 5,000 years ago.

The idea that amylase has experienced intense natural selection in certain regions of the world may sound like “a remarkable and kind of crazy claim,” Dr. Sudmant said. “But maybe it’s not so crazy, because this is a region of the genome that’s both plastic and powerful.”

Amylase still poses profound mysteries. Today, the amount of enzyme any given individual makes seems to have no major effects on health or reproductive success. If there’s no advantage to extra amylase genes today, what sort of advantage could they have provided our ancestors?

To answer that question, we may have to rethink the role of amylase in our mouths, Dr. Gokcumen said. Researchers have traditionally thought of the enzyme as the first step in our digestion of starch. But perhaps its real job is to provide our bodies with a signal that food is on its way.

If that’s true, then more amylase might prompt people to make more insulin, which would in turn make them absorb more sugar from starch, he said.

In ordinary times, this signal might not have mattered much. But in times of famine, it might have ensured that people could get the most out of what little food they could find.

“If you have lots of bread around, there’s no problem,” Dr. Gokcumen said. “But if you if you’re just barely surviving, then I think it will be a matter of life and death.”

George Perry, an anthropological geneticist at Pennsylvania State University who was not involved in the studies, cautioned that intense natural selection on the amylase gene might not be the only explanation for how it became so common in Europe and western Asia.

From time to time, large groups arrived from elsewhere, abruptly replacing the people who had lived there for thousands of years. The newcomers might have simply brought extra copies of the amylase genes with them.

“I’m really excited by both these papers, but I would stop short of saying it’s definitive evidence,” he said.

Translation

早期人類如何進化到吃澱粉 (2/2)

兩項新研究發現，古人類祖先帶有澱粉酶（一種分解澱粉的酵素）基因的多樣性令人驚訝。

（繼續）

早期的狩獵採集者經歷了DNA突變，在他們的 DNA 中添加了更多的澱粉酶基因。有些人經歷基因被砍掉而導致份額數目比父母少的基因突變。但新研究並沒有發現任何證據顯示狩獵採集者因擁有額外的澱粉酶基因而獲得任何演化優勢。

新研究發現，大約 12,000 年前，情況發生了巨大變化。正是在最後一個冰河時期末期，許多社會開始自種農作物，包括小麥、大麥和馬鈴薯等富含澱粉的食物。

在歐洲和西亞，考古學家發現了這段時期的大量骨骼，遺傳學家從中提取了 DNA。新的研究表明，在過去的 12,000 年裡，含有額外澱粉酶基因的 DNA 更加普遍。

科學家得出的結論是，對這一趨勢的最佳解釋是自然選擇的作用：澱粉酶基因較多的人比澱粉酶基因較少的人更有可能生存並生育孩子。

兩個團隊都發現了更多證據，表明澱粉酶不僅在歐洲和西亞得到自然選擇帶給的十分有利位置，而且在人們轉向富含澱粉飲食的其他地方也是如此。

在一項尚未發表的研究中，Gokcumen 博士的研究小組發現，在秘魯，澱粉酶的額外份額數目在過去幾千年中迅速增加，而在那處馬鈴薯在 5000 多年前就成為家種農作物。

Sudmant博士說，澱粉酶在世界某些地區經歷了強烈的自然選擇的想法聽起來可能是「一個非凡且瘋狂的主張」; 「但也許這並沒有那麼瘋狂，因為這人的基因組區域既是有性可塑性又是強勁的」。

澱粉酶仍然是一個深奧的謎團。如今，酵素數量對任何個別人仕的健康或成功生殖沒似乎有重大影響。如果今天額外的澱粉酶基因沒有優勢，那麼它們能為我們的祖先提供什麼樣的優勢呢？

Gokcumen博士說，為了回答這個問題，我們可能必須重新思考澱粉酶在我們口腔中的作用。研究人員傳統上認為酵素是我們消化澱粉的第一步。但也許它真正的作用是向我們的身體發出食物正在來臨的信號。

他說，如果這是真的，那麼更多的澱粉酶可能會促使人們產生更多的胰島素，這反過來又會讓他們從澱粉中吸收更多的糖。

在平時，這個訊號可能影響不大。但在飢荒時期，它可能可以確保人們能用盡他們所能找到的少量食物。

Gokcumen博士說: 「如果你身邊有很多麵包，那就沒問題」; 「但如果你只是勉強生存，那麼我認為這將是生死攸關的問題」。

賓州州立大學的人類學遺傳學家George Perry沒有參與這項研究，他警告說，對澱粉酶基因的強烈自然選擇, 並不是可能令澱粉酶在歐洲和西亞如此普遍的唯一解釋。

不時有大群人從其他地方湧來，突然取代了在那裡生活了數千年的人。是新來者可能帶來了額外的澱粉酶基因數目。

他說:「我對這兩篇論文感到非常興奮，但我不會說這是絕對肯定的證據」。

              So, according to this article, the enzyme known as amylase was critically important for the evolution of our species as we adapted to a changing food supply. Two new studies reveal that our ancestors began carrying more amylase genes in two major waves: the first one several hundred thousand years ago, possibly because of humans starting to cook with fire, and the second after the agricultural revolution 12,000 years ago. As people depended more on starch in their diet, this paper suggests that natural selection might have favored those who made more amylase in their mouth. The extra enzymes might have also helped them absorb more nutrition.

Note:

1. Hunter-gatherers (狩獵採集者) or forager is a human living in a community, or according to an ancestrally derived lifestyle, in which most or all food is obtained by foraging, that is, by gathering food from local naturally occurring sources, especially wild edible plants but also insects, fungi, honey, bird eggs, or anything safe to eat, and/or by hunting game. Hunting and gathering was humanity's original and most enduring successful competitive adaptation in the natural world. Following the invention of agriculture, hunter-gatherers who did not change were displaced or conquered by farming or pastoralist groups in most parts of the world. (Wikipedia)

2. Natural selection (自然選擇) is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. (Wikipedia)