The earliest living organisms - single, free-living cells - could have depended for their existence on structures that were very simple compared with the highly evolved structures we see today.

The cell membrane, for example, has sophisticated pumps, channels and pores for moving materials in and out of the cell. Researchers at Massachusetts General Hospital, writing in the journal Nature, say the most primitive cells made do with simple membranes of fatty acids. When these chemicals are concentrated in water, they assemble into membranes, which then close to form tiny spheres.

The researchers say such membranes would allow small molecules to enter the cell while large molecules, such as the genetic materials DNA and RNA, would be trapped.

How these simple membranes evolved into today's complex structures is unknown but it is clear the distinction between living and non-living was very fine when the world was young.

作为地球上最早的生物，单体自由细胞为了存活，可能依赖于一种非常简单的结构，远不及我们今天所见到的高度进化的结构。

例如，细胞膜具有复杂的泵、通道以及气孔，用于输送物质进出细胞。在《自然》(Nature)杂志刊登的一篇论文中，美国马萨诸塞州总医院(Massachusetts General Hospital)的研究人员写道，制造最原始的细胞利用的是简单的脂肪酸膜。当这些化学物质在水中聚集的时候，它们就会集合成膜，接着这些膜就近乎形成微小的球体。

研究人员表示，这种膜会允许小分子进入到细胞之中，而像基因物质DNA和RNA等大分子则会被阻留在细胞之内。

尚未清楚这些简单的膜是如何进化成今天的复杂结构的，不过有一点很清楚，在生命之初，生物与非生物之间的差别非常微小。
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