繁殖[e 9]係有生命嘅嘢最重要嘅特徵之一,而基因[e 10]係所有已知生物將自己嘅特徵傳俾下一代嘅基本媒介:每隻生物嘅細胞裏面都有啲 DNA 份子喺度,呢啲 DNA 份子成雙螺旋[e 11]噉嘅結構,一條 DNA 裏面有能力決定隻生物嘅特徵嘅某個特定區域就為止一個基因,例如想像人嘅 DNA,如果 DNA 當中嘅某一段決定人嘅頭髮顏色-嗰段有 同 兩個唔同嘅可能款;身上有 嘅人會係金頭髮,而身上有 嘅人會係黑頭髮-噉呢段 DNA 就算係決定頭髮顏色嘅基因[26][27]。
基因決定咗一隻生物嘅遺傳資訊:一隻生物內部會靠 DNA 嘅基因產生相應嘅 RNA 基因,而由呢啲 RNA 基因嗰度就會產生出蛋白質[e 12],跟手呢啲蛋白質就會俾隻生物攞嚟組成身體嘅各種構造,例如動物嘅肌肉以至神經細胞都係要有蛋白質先組成得到;所以一隻生物身上嘅 DNA 會影響佢嘅各種特徵,由肌肉嘅大細(影響佢大唔大力)以至個腦嘅神經細胞嘅數量(影響佢嘅智能同各種認知特性[28])等。好多時,就算係極之唔同嘅生物物種身上都會有相同嘅基因,顯示咗唔同嘅生物好多時都需要相同嘅身體功能,舉個例子說明,如果將喺人類當中負責產生胰島素[e 13]嘅基因植入去第啲生物嗰度嗰時,個基因都仲係會扮演生產胰島素嘅功能[26]。
喺廿一世紀初嘅無生源論上,「有 DNA 先定有蛋白質先」係一條受關注嘅問題:傳統定義上嘅生物冚唪唥都有能力整蛋白質-蛋白質係一大類由氨基酸組成嘅聚合物[e 56],對好多生命過程都不可或缺。而喺生物體內控制啲蛋白質嘅合成嘅就係 DNA:一條 DNA 上面嗰啲基因決定咗隻生物嘅身體會合成邊一啲種類嘅蛋白質,而唔同種嘅蛋白質整出嚟嘅結構會唔同,所以 DNA 間接決定咗隻生物嘅身體構造同特徵;另一方面,基因嘅形成又係要有蛋白質先搞得掂嘅。於是乎就有個「有 DNA 先定有蛋白質先」嘅問題,就好似詏緊「有雞先定有蛋先」噉[120][121]。廿一世紀嘅生物學目前基本上肯定,基因同蛋白質唔係獨立噉各自出現嘅[122]。
為咗解決「有 DNA 先定有蛋白質先」呢條問題,英國生物學家法蘭西斯·卡里卡[e 57]提出咗 RNA 世界[e 58]嘅諗法:RNA 都係一種聚合物,喺蛋白質合成嘅過程中途會出現並且轉化做第啲嘢,最後產生蛋白質;RNA 好似 DNA 噉能夠儲起啲資訊,又好似某啲蛋白質噉能夠催化某啲化學反應(後者呢一點喺 1986 年由美國一位化學家驗證咗[123]),即係大致上可以諗成:
結合呢一柞研究嘅結果:早期地球上面嗰啲 DNA 同 RNA 唔一定要係喺地球上面產生嘅;於是有科學家就諗,有生命嘅嘢(或者至少生命必要嗰啲化合物)可能係喺太空嗰度產生,然後跟手坐住啲小行星飛去一啲啱生物生存嘅行星嗰度發展[135]。再廣義化啲噉睇,宇宙當中好可能有大量帶有有機化合物嘅小行星喺太空裏面飛嚟飛去,將生命帶去宇宙嘅各個角落[127]。
定義死亡喺醫學上係一條有些少挑戰性嘅問題:定義上,死亡係「生命嘅終結」,但正如上面提咗,「生命」呢個詞要點定義已經係一個令人頭痕嘅問題,所以定義一隻生物究竟「死咗未」或者斷定佢嘅「死亡時間」亦唔係容易嘅事;例如一隻動物死嗰陣,好多時佢身體各個器官死嘅時間都唔同,例如個肝嘅細胞可能喺個腦死咗之後幾分鐘先至死得嗮[173]。一般嚟講,醫學上會用「心跳停咗」或者「個腦死咗」嚟當做由生到死之間嘅界線;但有陣時,啲腦死嘅病人又可以靠架生幫手做唞氣同心跳呢啲基本功能[171],而且廿一世紀初嘅醫學界已知有瀕死體驗(NDE)嘅現象-喺 NDE 期間,病人嘅腦部好多時經已停止運作(用腦死做死亡定義嘅話算係經已死咗),不過病人後嚟救得返,而且仲能夠正確噉描述佢「死咗」期間發生嘅事,又引起咗生物學界同醫學界對死亡嘅討論[174][175]。
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Richard Dawkins > Quotes > Quotable Quote. Goodreads,原句:"The total amount of suffering per year in the natural world is beyond all decent contemplation. During the minute that it takes me to compose this sentence, thousands of animals are being eaten alive, many others are running for their lives, whimpering with fear, others are slowly being devoured from within by rasping parasites, thousands of all kinds are dying of starvation, thirst, and disease."