酸素といえば、有用な元素であるイメージで多く使われます。呼吸では酸素を吸収するし、ものを燃やすのも酸素の働きです。
しかし一方で、酸素には人にとっても「嫌な」反応も起こします。学校の化学の授業でならった酸化という反応を覚えている人がいるでしょう。金属を黒ずませたり、食べ物を腐らせたりするのは、物質と空気中の酸素が結びつく酸化が原因です。
原始地球には酸素は存在していませんでした。つまり、そのころに地球上に生きていた原始の生物にとって、酸素は自身の生命を脅かす「毒ガス」だったのです。初期の生物は 嫌気性生物 だったと考えられています。
その後、現在の生物でいう シアノバクテリア のような植物が現れ、二酸化炭素から酸素が生み出されて大気へ放出されるようになってはじめて、地球の大気に酸素が放出されることになりました。
そして、その「毒ガス」によってたくさんの生物が死んでしまったと考えられます。そして、逆にその「毒ガス」に晒されても死なない生物は生き残り、また「毒ガス」を利用することができる好気性細菌も出現しました。しかも、これが 嫌気呼吸 よりもはるかに効率が良いエネルギー生成方法だったのです。
酸素を利用はしないが大気中でも生存できる生物は耐酸素性細菌と呼ばれる生物もいます。このような生物が好気性細菌を食べたとき「そうだ、利用してはどうだろうか」と思ったのです(実際はそんなことを考える脳は発達してはいませんでしたが)。
「こいつ、有害な酸素を吸収してくれるし、しかもエネルギーまで生産してくれる。消化しないでパシリにしてやろう」
こうして、共生することに成功した生物は、他の生物よりもより環境に適応し、自然選択で生き残ってきたのです。その一部が、哺乳類、人間にまで受け継がれているのです。
好気性細菌は、宿主によっていいように「パシリ」にされたとも考えられますが、好気性細菌のほうも宿主の中にいることで、自分は危険に晒されることはありません。栄養がたっぷりある宿主の細胞の中に漂っていれば生存できる。「パシリ」も悪くないなと思ったことでしょう(実際に思うことはなかったと考えられますが)。
わたしたち人間の身体の中にも、この好気性細菌の末裔がうじゃうじゃしています。現在は 真核生物 の細胞ではミトコンドリアという名前がつけられていますが、それは一個の細胞に数百から数千もあるのです。
ミトコンドリアなしには、わたしたちは身体の動かしたり消化したりと生命活動をおくるエネルギーを得ることができません。自分にとって「害のあるもの」を逆に利用することがが、自然選択、つまり厳しい社会の淘汰を生き抜く秘訣なのではないでしょうか。
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