「生きている」とはどういうことか? 「生物」の定義

悩みを抱えたとき、生物学的に考えるとすんなりと答えが見つかったり、悩みが解決したりすることがあります。それはわたしたち人間が「生物」「自然」の一端であるからでしょう。

ここでは、悩んだとき、大切な決断をするときなどに、生物学的問題に立ち返ることでそれを解決するヒントを書いていきたいと思います。原点に人間や文明は「自然」の真逆にあるものと考えられることがありますが、人間そのものは紛れもない自然の一部なんですよね。

生きている生物の特徴とは

「生きている」ことはどういうことか、しっかりと明らかにしなければ話は先に進めません。まずは「生きている」とはどういうことかをしっかりと定義してみましょう。

生きている生物の特徴は、大きく2つに分かれます。

ひとつは「自己を維持することができる」ということです。つまり、食べて栄養を取り込み、酸素を吸って、栄養を分解して、身体やそれを構成する細胞がその存在を維持することができるということです。「代謝・成長」と呼ばれる特徴です。

また、外から刺激が加えられたときは、それに適切に反応して、生命を維持しなければなりません。沸騰したお湯が入ったヤカンに触って、「熱い」という刺激が受け取られても、そのまま触れ続けている人はいないですよね。そんなことをしていたら、手が大やけどしてしまって、手のやけどした部分の細胞はすべて死んでしまいます。

やけど程度なら死ぬのは細胞の一部だけなのでまだよいですが、もし「熱い」のが火事だったら命に関わってしまいます。このように生物が刺激に反応することは「刺激反応性」と呼ばれます。

また、「お腹が空いた」という内部からの刺激に反応して、ご飯を食べるというのは、前者の「代謝・成長」と後者の「刺激反応性」の両方が合わさって、自己を維持することができているといえるでしょう。

しかし、その両方が合わさっても永遠に生きていることを維持することはできません。ヒトの寿命はだいたい最長で120歳、昆虫や草木には一年だけのものが多いですし、本当に数日の命の生物もいます。しかし、寿命がきたらすべて死んでしまっては、その種はすぐに絶えてしまいます。

そこで、生きている生物の特徴のふたつ目、「自分のコピーを作る」という特徴です。

生物は自分と同じ性質の生物を増殖させる働きを持っています。いくつかの植物が芋を植えたり、差木をしたら増えるように、まったく同じ個体を増殖させることができます。また、ヒトを含む哺乳類・鳥類・爬虫類のように有性生殖を行う種は、半分だけは自分と同じ性質の個体を増殖させることができます。

親から子へ、形質が受け継がれることを、これを「遺伝」といいます。有性生殖をする種においては、親と子は似てはいるけれど、まったくコピーではないですよね。しかし、性細胞以外の細胞(体細胞)はすべて、まったく自分自身と同じコピーを作りつづけているのです。もし違うものができてしまった場合、それは癌と呼ばれ、悪性ならば病気になってしまいます。

「生きている」とは、
・ 自己を維持することができる
・ 自分のコピーを作ることができる
ということ。

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