Q-Eng

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生命の起源は多くの仮説がある。 このTED-Ed は 従来から有力視されているもので、
深海の熱水噴出孔の近辺で 無機物や有機物から生命が誕生したという話。
発話速度はかなり遅く、内容は簡単だが、生物学用語が少し出てくる。

中級　４分　140wpm　　　　　　　　　　　　　　　　　　　　　　　　　2019年 　　

字幕 ： 開始後 で字幕On/Off､ で言語選択。文字の色やサイズﾞはｵﾌﾟｼｮﾝから｡
.　　　　 動画を見るとき、 でフルスクリーンに拡大すると見やすい。

★下記英文は ポップアップ辞書が使えます｡
　　テキストはこちら⇒日英トランスクリプト （字幕はYouTubeの方が大きく見やすい）　　　　　　　

Billions of years ago on the young planet Earth simple organic compounds assembled into more complex coalitions （複合） that could grow and reproduce. They were the very first life on Earth, and they gave rise to every one of the billions of species that have inhabited our planet since.
　Earth ： 地球は天文学の固有名詞としては Earth 。（無冠詞、先頭が大文字）

At the time, Earth was almost completely devoid（欠けている）of what we’d recognize as a suitable environment for living things. The young planet had widespread volcanic activity and an atmosphere that created hostile conditions.

So where on Earth could life begin? To begin the search for the cradle（揺籃の地） of life, it’s important to first understand the basic necessities for any life form. Elements and compounds essential to life include hydrogen, methane, nitrogen, carbon dioxide, phosphates（リン酸）, and ammonia.

In order for these ingredients to comingle and react with each other, they need a liquid solvent: water. And in order to grow and reproduce, all life needs a source of energy. Life forms are divided into two camps: autotrophｓ （独立栄養生物）, like plants, that generate their own energy, and heterotrophs（従属栄養生物）, like animals, that consume other organisms for energy.

The first life form wouldn’t have had other organisms to consume, of course, so it must have been an autotroph, generating energy either from the sun or from chemical gradients. So what locations meet these criteria? Places on land or close to the surface of the ocean have the advantage of access to sunlight.

But at the time when life began, the UV radiation on Earth’s surface was likely too harsh for life to survive there. One setting offers protection from this radiation and an alternative energy source: the hydrothermal（熱水の） vents that wind across the ocean floor, covered by kilometers of seawater and bathed in complete darkness.

A hydrothermal vent is a fissure（亀裂） in the Earth’s crust where seawater seeps into magma chambers and is ejected back out at high temperatures, along with a rich slurry（懸濁液） of minerals and simple chemical compounds.

Energy is particularly concentrated at the steep chemical gradients（勾配） of hydrothermal vents. There’s another line of evidence that points to hydrothermal vents: the Last Universal Common Ancestor of life, or LUCA for short. LUCA wasn’t the first life form, but it’s as far back as we can trace.
　LUCA ： Last Universal Common Ancestor 最後の共通祖先
　　　　　　46億年の地球史で私達がたどれる最古の生命（約40億年前の単細胞生物）

Even so, we don’t actually know what LUCA looked like— there’s no LUCA fossil, no modern-day LUCA still around— instead, scientists identified genes that are commonly found in species across all three domains of life that exist today.
　three domains ： 現存する生物の３領域 (細菌、真核生物、古細菌) 　　　　　　

Since these genes are shared across species and domains, they must have been inherited from a common ancestor. These shared genes tell us that LUCA lived in a hot, oxygen-free place and harvested energy from a chemical gradient— like the ones at hydrothermal vents.

There are two kinds of hydrothermal vent: black smokers and white smokers. Black smokers release acidic, carbon-dioxide-rich water, heated to hundreds of degrees Celsius and packed with sulphur, iron, copper, and other metals essential to life. But scientists now believe that black smokers were too hot for LUCA— so now the top candidates for the cradle of life are white smokers.

Among the white smokers, a field of hydrothermal vents on the Mid-Atlantic Ridge called Lost City has become the most favored candidate for the cradle of life. The seawater expelled here is highly alkaline and lacks carbon dioxide, but is rich in methane and offers more hospitable temperatures.

Adjacent（近くで） black smokers may have contributed the carbon dioxide necessary for life to evolve at Lost City, giving it all the components to support the first organisms that radiated into the incredible diversity of life on Earth today.
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