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The Origins of Multicellularity(多細胞生物的起源)

The Origins of Multicellularity

Sponges are the simplest multicellular animals. Because cells are the elementary units of life, organisms larger than unicellular organisms arose as aggregates of such building units. Nature experimented with producing larger organisms without cellular differentiation—certain large, single-celled marine algae, for example—but such examples are rarities. Multicellularity has many advantages over increasing the mass of a single cell. Because cell surfaces exchange molecules with the environment, dividing a mass into smaller units greatly increases the surface area available for metabolic activities.

A workable surface-to-mass ratio cannot be maintained by increasing the size of a single-celled organism. Thus multicellularity is a highly adaptive path toward a larger body size.

Strangely, while sponges are multicellular, their organization is quite distinct from that of other metazoans. A sponge body is an assemblage of cells embedded in an extracellular matrix and supported by a skeleton of minute needlelike spicules and protein. Because sponges neither look nor behave like other animals,they were not completely accepted as animals by zoologists until well into the nineteenth century. Nonetheless, molecular evidence demonstrates that sponges are phylogenetically grouped with other metazoa (animals).

多細胞生物的起源

海綿是最簡單的多細胞動物。因為細胞是生命的基本單位,高于單細胞生物的生命形式以這種單位聚集的形態出現。自然嘗試著以一種不進行細胞分化的方式生產更大的生物——例如某些大的、單細胞藻類——但是這些例子很罕見。多細胞生物具備很多優點,遠遠超過單純是單一細胞的聚集。因為細胞表面分子與環境交換時,把細胞分割成更小的單位,可以大大增加可用的代謝活性表面積。一個有用的表面積-質量比,不能通過增加單一細胞有機體的體積來維持。因此,對于一個更大的生命個體而言,多細胞是一種達成高度適應的途徑。

奇怪的是,盡管海綿是多細胞組織,它們的組織卻與其他后生動物完全不同。海綿的身體是由嵌入外基質的細胞、細小的針狀骨針和蛋白質骨架組合而成的。因為海綿無論從外形還是行為上都不像其他動物,所以直到19世紀之前,它們并沒有被動物學家完全接受。然而,分子證據表明,海綿與其他后生動物(動物)在進化史上是歸為一類的。

生物131 李唯學號1302040115

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