If all humans share common origins, it seems reasonable to suppose that cultural diversity could also be traced to more constrained beginnings.
【重点词汇】origins 起源 reasonable 合理的 diversity多样性 constrained有局限性的
【结构分析】句子主干：it seems reasonable to suppose that为it形式主语，it不译，其他顺译即可
修饰成分：if引导条件状语从句，that引导的是宾语从句
【结构分析】如果所有人类有共同的起源，似乎有理由去认为文化多样性可能被追溯到更为局限的开端。
