人類陰莖海綿體的演進，可以從 "羊膜動物" 類群的胚胎發育觀察開始，在脊索動物門中，羊膜動物 "amniota"，是一群四足脊椎動物，包括合弓類動物 (哺乳類與似哺乳爬行動物) 與蜥形類 (含爬行動物、鳥類)。羊膜動物藉由產卵、胎生等方式繁衍，胚胎由多層膜來保護。在真獸類哺乳動物，更演化出羊膜來包覆胎兒。羊膜動物與兩棲動物的差異在於保護胚胎的膜，與缺乏幼體變形為成體的階段。因為自羊膜動物開始，產生了多樣性變化，例如從幾個代表性生物身上可以一窺端倪，像喙頭蜥 (tuatara) 在胚胎期本來有陰莖，但後來發育卻自行消失、針鼴 (echidna) 有四個陰莖、鴨子 (duck) 的陰莖長度是身長的二倍多，為甚麼會有這樣的差異呢？其產生變化的道理何在？
根據目前科學家的推測，脊索動物，包括人類，陰莖會有各種形變與演化、縮伸，多半與該動物的：1. 交配成功率 (受雄性打鬥死傷、環境天候危險、或交配季節長短等因素影響)、2. 受精著床容易度、3. 胚胎或幼體發育情形有關。例如：體內受精者，才需要陰莖這個器官；針鼴為求百分之百受精成功，具有四具陰莖；鴨子的陰莖極長，因為母體輸卵管彎曲不易著床，且在交配中常有其他雄性干擾與打鬥，導致體內受精失敗，故需極長，方不致鬆脫。
至於為何陰莖是 "海綿體"、而非橫紋肌、平滑肌、甚或骨骼呢？試著窺探造物者的想法，原因應與 "功能必要性" 有關；交配或受精行為，在體內受精動物雄性方而言，其目的乃將精液輸送至卵子處，所需的傳遞工具不必過於堅硬或強韌，故不致於發展到 "肌肉" 的等級。再者，交配行為不同於一般運動，需要強力支撐性能，故不需 "骨骼"。第三，交配行為不同於呼吸、與其他經久持續性的運動需要，陰莖不用於受精時，僅排尿而已，若始終保持堅硬，將導致其他行動的阻礙與空間上的浪費，為便於收藏計，故設計為 "縮伸"，可大可小，易於保管收納，適合間或使用。那麼，海綿體會是眾多體組織中不錯的選擇。
所以結論是：為求成功繁衍下一代，生命體自會尋求一更佳的發育方式來應應環境的變化與干擾，使生命不斷延續。
參考文獻與相關論述根據如下：
The diversity of penis features among amniote vertebrates has left evolutionary biologists with an unanswered question: Did male external genitalia evolve more than once? In a study published this week (October 28) in Biology Letters, researchers looked to embryos of an ancient amniote lineage, finding that they go through a developmental stage with a penis that’s conserved among other amniotes.
The subject of the study, a reptile called a tuatara, does not have a phallus as an adult. So looking at its development could lend insight into whether it experiences stages of genital growth common to other amniote lineages.
It's got a third eye complete with lens and retina, and despite its appearance isn’t actually a lizard. But these aren't the only odd things about the tuatara reptile: It also lacks a penis. Paradoxically, it's this fact that might just hold the clue as to how the organ evolved for the rest of us. Understanding whether the tuatara originally had a penis and then lost it could answer a problem that has been bothering biologists for a while now. Do we see such a massive diversity in animal penises because they all evolved independently, or did the penis evolve only once and then subsequently diversify?
The tuatara is the last survivor of a group of reptiles that flourished around 200 million years ago, but is now restricted to a few islands off the coast of New Zealand. Among a myriad of weird traits, this group is notable because out of all animals that require internal fertilization, or amniotes, it is the only one in which all members lack penises. In comparison, the rest of the male amniotes show an impressive amount of diversity when it comes to their sex organ. From the four-headed penis of the echidna, the permanently erect ones of the alligator, or the improbably long corkscrew of the Argentine lake duck, there seems no end to the variety.
This has led some to speculate that the erectable phallus didn't evolve once, but that actually each group within the amniotes – mammals, lizards and snakes, crocodiles, turtles and birds – all came up with their own solution to getting sperm to the eggs within the females. One way to solve this mystery would be to look at tuatara embryos, and see if in the very early stages of life the reptile develops the beginnings of a penis which it then gets rid of by reabsorption.
But there's a catch: the reptiles breed incredibly slowly. In fact, females only become sexually receptive at around 10-15 years of age, and even then only lay eggs around every 5 years. This means that New Zealand is obviously very protective of all tuatara embryos, not letting any go spare. Things, however, were a little more lax at the start of the 20th century, and it turns out that four of the reptiles' embryos were prepared back then in thin slices at Harvard. Getting their hands on these rare specimens, the researchers were able to photograph and then reconstruct a 3D model of the early embryos.
What they found seems to settle the debate. There was indeed a small nubbin exactly where the penis develops in all other amniotes, along with what would eventually go on to form the cloaca, a multi-purpose opening for urination, defecation, and reproduction. In fact, the genital swelling they observed looked a lot like what is seen in bird embryos, most of which also develop the beginning of penis then take it all back in. The results are published in Biology Letters.
This suggests that the penis did actually only develop once in amniotes, and some animals then went on to lose theirs over time, while others developed elaborate spines, swellings, and scrapers.
adapted and transcribed from:
http://www.the-scientist.com/?articles.view/articleNo/44380/title/Evolution-of-the-Penis/
http://www.iflscience.com/plants-and-animals/penis-less-reptile-unravels-mystery-phallic-evolution/
http://scienceblogs.com/pharyngula/2006/06/09/penis-evolution/
http://news.softpedia.com/news/Sex-War-and-Promiscuity-Why-Ducks-Have-the-Largest-Penises-in-the-World-53558.shtml
留言列表
神學一 (3)