The hypothesis of a connection between the onset (or intensification) of Northern Hemisphere Glaciation, the stepwise increase in African aridity (and climate variability), and an important mammalian (including hominin) species turnover is a textbook example of the initiation of a scientific idea and its propagation in science. It is, however, also an example of the persistent popularity of a hypothesis despite mounting evidence against it. A critical review of key publications on the topic and statistical re-analysis of key records of global ice volume and African climate leads to three conclusions: (1) The Northern Hemisphere Glaciation was a gradual process occurring between ∼3.5 and 2.5 Ma, not a single event at ∼2.8 Ma or at any other time. (2) A consistent stepwise (+/−0.2 Ma) transition toward greater aridity in Africa at ∼2.8 Ma does not exist; instead, there are regionally different, gradual transitions partly in connection with the intensification of the Northern Hemisphere Glaciation, but above all with the establishment of the tropical Walker Circulation after ∼2 Ma. (3) Mammalian (including hominin) species turnovers at this time also appear to have been gradual, rather than stepwise.

北半球冰川作用的开始（或加剧）、非洲干旱（和气候变异）的逐步增加以及重要的哺乳动物（包括古人类）物种更替之间存在联系的假设是科学思想启蒙的教科书例子及其在科学中的传播。然而，这也是一个假设持续流行的例子，尽管有越来越多的证据反对它。对该主题的关键出版物进行批判性审查，并对全球冰量和非洲气候的关键记录进行统计重新分析，得出三个结论：(1) 北半球冰川作用是发生在 ~3.5 到 2.5 Ma 之间的一个渐进过程，而不是〜2.8 Ma或任何其他时间的单个事件。(2) 持续的逐步 (+/-0.2 Ma) 过渡到非洲更大的干旱，大约为 2。8 马不存在；相反，存在区域性不同的渐进过渡，部分与北半球冰川作用的加剧有关，但最重要的是与大约 2 Ma 之后热带沃克环流的建立有关。(3) 哺乳动物（包括古人类）物种的更新此时也似乎是渐进的，而不是逐步的。