The Origins of Multicellularity(多细胞生物的起源)

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).

多细胞生物的起源

海绵是最简单的多细胞动物。因为细胞是生命的基本单位，高于单细胞生物的生命形式以这种单位聚集的形态出现。自然尝试着以一种不进行细胞分化的方式生产更大的生物——例如某些大的、单细胞藻类——但是这些例子很罕见。多细胞生物具备很多优点，远远超过单纯是单一细胞的聚集。因为细胞表面分子与环境交换时，把细胞分割成更小的单位，可以大大增加可用的代谢活性表面积。一个有用的表面积-质量比，不能通过增加单一细胞有机体的体积来维持。因此，对于一个更大的生命个体而言，多细胞是一种达成高度适应的途径。

奇怪的是，尽管海绵是多细胞组织，它们的组织却与其他后生动物完全不同。海绵的身体是由嵌入外基质的细胞、细小的针状骨针和蛋白质骨架组合而成的。因为海绵无论从外形还是行为上都不像其他动物，所以直到19世纪之前，它们并没有被动物学家完全接受。然而，分子证据表明，海绵与其他后生动物（动物）在进化史上是归为一类的。

生物131 李唯学号1302040115