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Scientists have been debating this question for a long time.
科学家们围绕这个问题已经争论了很长的时间。
Some believed that snakes evolved on land,and that their ancestors were small,burrowing lizards.
一些科学家认为蛇从陆地上进化而来，它们的祖先是小的，穴居的蜥蜴。
And others believed that snakes evolved fromthe mosasaurs, giant swimming lizards that lived 144 to 65 million years ago.
另一些科学家认为蛇由沧龙进化而来，沧龙是生活在一亿四千四百万年前至六千五百万年前的一种巨型水生蜥蜴。
Except it's difficult to analyze the genes of the extinct mosasaurs.
分析已经灭绝的沧龙的基因是非常困难的。
So instead, scientists at Penn Stateanalyzed the genes of their closest living relatives, the monitor lizards.
所以宾夕法尼亚州立大学的科学家们分析了它们现存的最近亲属巨蜥的基因。
In fact, these scientists examinedgenetic material representing all nineteen living lizard families, and seventeen of the twenty-five familiesof living snakes.
实际上，这些科学家们检查了代表所有19种现存蜥蜴家族以及蛇的25种现存家族中的17种基因材料。
They analyzed the differences in genetic structure that inevitably result from the slight mutations that develop over time.
他们分析了随着时间的推移，遗传结构中不可避免的微小变化所带来的差异。
This enabled them to map a genetic family tree.
这让他们能够绘画出一个基因家谱图。
They discovered that snakes aren'tclosely related to monitor lizards.
他们发现了什么呢？他们发现了蛇与巨蜥并没有密切的关系。
Now, because the mosasaurs– the monitor lizards' ancestors–werethe only marine lizards around when snakes evolved, snakes must have evolved from lizards living onland.
现在，因为沧龙-巨蜥的祖先-是蛇进化时周围唯一的海洋蜥蜴，所以蛇应该是从在陆地上生活的蜥蜴进化而来。
There's a strong correlation between the burrowing lifestyle and very small limbs or the complete lossof limbs.
从穴居生活到非常小的四肢，再到完全丧失四肢，这之间的关联非常强。
Some burrowing lizards have reduced limbs and some have lost one or both pairs.
一些穴居蜥蜴的四肢减少，一些已经失去了一个或者所有的四肢。
After all,you could fit into much smaller openings if you didn't need a pelvis or shoulders to support your limbs.
毕竟,你需要适应更小的通道，如果你不需要骨盆或肩膀来支撑你的肢体。
