Why are there no persisting hybrids of humans with Denisovans, Neanderthals, or anyone else?
Why are there no persisting hybrids of humans with Denisovans, Neanderthals, or anyone else?
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为什么人类与丹尼索瓦人、尼安德特人或其他人没有持久的杂交？
DOI：
10.1073/pnas.1602270113
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发表时间：
2016
影响因子：
11.1
通讯作者：
Varki,Ajit
中科院分区：
文献类型：
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作者：
Varki,Ajit
索耶等人的巡回报告。(1)对两个丹尼索瓦人基因组的研究以及伴随的社论和图（2）支持了“一个由有限但间歇性的......基因流”（3）：数千年来，在除了我们“行为现代人”（BMH）之外的所有分类群神秘消失之前，涉及多个人类谱系。虽然人们的注意力集中在非BMH等位基因的罕见渗入上，这些等位基因促进了入侵BMH对生态挑战的适应，但房间里还有一只更大的大象。目前的基因组和考古学数据表明，BMH在10万至20万年前出现在非洲，并遍布全球（包括非洲其他地区），遇到了其他现存的人类，如尼安德特人，丹尼索瓦人，古代非洲人，以及可能来自早期直立人散居地的其他血统。虽然基因组证据表明杂交，但功能基因的数量有限，导致“渗漏替代”（3），没有真正的杂交后代。因此，我们的单一BMH（亚）物种是每一个接触/更换事件的“赢家”，跨越数万年。我找不到任何其他的例子，其中一个单一的（亚）物种从一个地理起源完全取代了所有现存的交叉可育（亚）物种在每一个行星的位置，与有限的渐渗功能的遗传物质从取代分类群，并没有留下杂交物种。通常情况下，人们会发现多个可杂交的（子）物种，中间有杂交区。虽然这种明显的独一无二的现象可能是偶然发生的，但这种奇异性使人们能够解释BMH独特复杂的遗传/生物/文化转变。正如帕博（Pääbo）所指出的（3），遗传变异（等位基因）的“爆炸性星座”的适应性积累可能赋予了BMH无与伦比的认知特征组合，保证了在随后的每一次与其他古人类的相遇中取得成功。为什么杂交物种没有持续存在，至少在BMH扩张的地理极端？假设数百个新的等位基因组成了BMH基因型，与其他人类的F1杂交后代可能缺乏在BMH群体内竞争交配所需的完整认知能力。值得注意的是，10个非BMH线粒体序列中有10个超出了当前的BMH范围（1），表明BMH雄性与非BMH雌性交配产生的后代不包括在BMH组中。相反，雌性BMH和非BMH雄性的后代可能有机会在BMH群体中生存，具有足够的交配成功率，可以传递一些对新来者有价值的等位基因，但与生态适应有关，而不是认知。这种“人类例外论”目前是不受欢迎的，因为是进化事件的非凡解释。然而，除非有其他明确的例子，这样完全取代所有相关类群由一个单一的（亚）种，BMH可能确实是一个罕见的例外。虽然环境因素，如气候或传染病（4）可能产生了最初的非洲瓶颈，关键的BMH表型可能是认知。这符合预测尼安德特人通过与现代人竞争灭绝的生态文化模型（5），并表明BMH通过长期存在的“心理进化障碍”进行了不太可能的转变-可能涉及最初的适应不良特征，如现实否认和死亡率突出，这共同产生了获胜的组合（6）。
The tour-de-force report of Sawyer et al.(1) on genomes of two Denisovans and the accompanying editorial and figure (2) support the notion of “a web of now-extinct populations linked by limited, but intermittent... gene flow”(3): involving multiple hominin lineages for thousands of years, before the mysterious disappearance of all taxa other than us “behaviorally modern humans”(BMHs). Although attention focuses on rare introgressions of non-BMH alleles facilitating adaptation of invading BMHs to ecological challenges, there is a bigger elephant in the room. Current genomic and archaeological data indicate that BMHs arose in Africa∼ 100,000–200,000 y ago and spread across the planet (including the rest of Africa), encountering other extant hominins like Neanderthals, Denisovans, archaic African hominins, and possibly other lineages from earlier diasporas of Homo erectus. Although genomic evidence indicates interbreeding, the number of functional genes incorporated is limited, resulting in a “leaky replacement”(3), without persistence of true hybrids. Thus, our single BMH (sub) species was the “winner” in every contact/replacement event, spanning tens of thousands of years. I cannot find any other example wherein a single (sub) species from one geographic origin completely replaced all extant cross-fertile (sub) species in every planetary location, with limited introgression of functional genetic material from replaced taxa, and leaving no hybrid species. Typically, one instead finds multiple crossfertile (sub) species, with hybrid zones in between. Although this apparent one-of-a-kind phenomenon could have occurred by chance, the singularity allows one to posit a uniquely complex genetic/biological/cultural transition of BMHs. As Pääbo suggested (3), adaptive accumulation of an “explosive constellation” of genetic variants (alleles) could have endowed BMHs with an unparalleled combination of cognitive features, guaranteeing success at every subsequent encounter with other hominins. Why did hybrid species not persist, at least at the geographical extremes of BMH expansion? Assuming that hundreds of new alleles comprised the BMH genotype, F1 hybrids with other hominins would likely lack the complete cognitive package required to compete for mating within BMH groups. Tellingly, 10 of 10 non-BMH mitochondrial sequences are outside the current BMH range (1), suggesting that mating of BMH males with non-BMH females generated progeny that were not included within BMH groups. In contrast, progeny of female BMHs and non-BMH males may have had the opportunity to survive within BMH groups, with sufficient mating success rates to allow transmissions of a few alleles valuable to the newcomers, but related to ecological adaptation, not cognition. Such “human exceptionalism” is currently frowned upon, as are extraordinary explanations of evolutionary events. However, unless there are other clear examples of such complete replacement of all related taxa by one single (sub) species, BMHs may indeed be a rare exception. Although environmental factors such as climate or infectious disease (4) could have generated the initial African bottleneck, the critical BMH phenotype was likely cognitive. This fits ecocultural models predicting Neanderthal extinction through competition with modern humans (5) and suggests an improbable BMH transition through a long-standing “psychological evolutionary barrier”––possibly involving initially maladaptive features such as reality denial and mortality salience, which conspired to generate the winning combination (6).