All this is problematic enough when you are the intellectual master of the planet,but when you are a small, vulnerable australopithecine, with a brain about thesize of an orange, the risk must have been enormous. Absolute brain size doesnot tell you everything-or possibly sometimes even much. Elephants and whalesboth have brains larger than ours, but you wouldn't have much troubleoutwitting them in contract negotiations. It is relative size that matters, apoint that is often overlooked. As Gould notes, A. africanus had a brain ofonly 450 cubic centimeters, smaller than that of a gorilla. But a typical africanus male weighed less than a hundred pounds, and a female much lessstill, whereas gorillas can easily top out at 600 pounds (Gould pp. 181-83).

即使在今天，当你已经成为这个星球的智力发达的主人时，这一切依然是个大问题。而对于矮小而又容易受到伤害的南方古猿来说，婴儿降生时的脑大约有一只柑橘那么大（注：诚然，脑的绝对大小并不能说明一切——有时甚至不能说明很多。大象和鲸的脑都比我们大，但是和它们相比你显然聪明许多。脑的大小只是一个相对重要的数据，一个常常可以忽略不计的数据。正如古尔德所指出的那样，南猿非洲种的脑只有450立方厘米，比大猩猩的要小。而一个典型的南猿非洲种体重小于45千克，女性则更轻，而大猩猩的体重则大都重达150多千克。），危险性肯定很大。

Sowhy did Lucy and her kind come down from the trees and out of the forests?Probably they had no choice. The slow rise of the Isthmus of Panama had cut theflow of waters from the Pacific into the Atlantic, diverting warming currentsaway from the Arctic and leading to the onset of an exceedingly sharp ice agein northern latitudes. In Africa, this would have produced seasonal drying andcooling, gradually turning jungle into savanna. "It was not so much thatLucy and her like left the forests," John Gribbin has written, "butthat the forests left them."

因此，露西和她的同类从树上来到地上，而后又走出非洲丛林，这是为什么？他们很可能别无选择。巴拿马地峡的慢慢上升阻断了太平洋的海水流进大西洋，改变了流向北极的暖流的方向，使得北纬地区出现了异常寒冷的冰川期。在非洲，季节性干燥和寒冷气候的出现逐渐使得森林变成了草原。“与其说露西和她的同类离开了森林，”约翰·格里宾写道，“不如说是森林离开了他们。”