In the crowded central regions of the galaxy, home to large Numbers of massive stars, supernovas are so common that the evolution of complex life-forms might be difficult if not impossible.
在银河系拥挤的中心区域存在着大量的大质量恒星，频繁的超新星爆发使得复杂生命形式的产生不是不可能就是极度困难。
Suppose the evolution of higher life forms takes another couple billion years, as it did here.
再假设更高级形式的生命要再花几十亿年才能进化，就如地球上所经历的那样。
Scientists of the time knew that evolution occurred. The fossil record showed evidence of life forms that no longer existed. The question was, how did it occur?
当时的科学家知道进化的发生。化石记录表明了不再生存的生物迹象。问题是，进化是怎样发生的？
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