Early humans went through a tall and skinny phase on the African savannahs 1.5 million years ago, new British research suggests.
A study of fossils spanning four million years found that the stature and bulk of our ancient ancestors did not change consistently over the course of evolution.
But scientists were surprised to find that around 1.5 million years ago human-like "hominins" grew 10cm taller without gaining any extra weight.
It took a further million years for them to put on another 10-15kgs.
"An increase solely in stature would have created a leaner physique, with long legs and narrow hips and shoulders," said lead researcher Dr Manuel Will, from Cambridge University.
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"This may have been an adaptation to new environments and endurance hunting, as early Homo species left the forests and moved on to more arid African savannahs.
"The higher surface-to-volume ratio of a tall, slender body would be an advantage when stalking animals for hours in the dry heat, as a larger skin area increases the capacity for the evaporation of sweat."
The later appearance of fatter bodies coincided with migrations to colder, higher latitudes, he added.
However, Dr Will pointed out that "vast gaps" in the fossil record made it hard to grasp the real story of human evolution.
His team often had to estimate body sizes from tiny fragments of remains, in some cases a single toe bone.
The shape of early precursor humans ranged from the broad, gorilla-like Paranthropus to the wiry Australopithecus afarensis.
Hominins from four million years ago weighed an average of 25kg and stood between 125cm and 130cm tall.
The appearance of our own species family, Homo, around 2.2 to 1.9 million years ago saw a surge in both height (20cms) and weight (15-20kg).
Shortly after the emergence of the species Homo erectus, humans attained the kind of height they have today, while their weight stayed the same.
Adaptations to habitats north of the Mediterranean led to heavier "people" appearing in the fossil record between 400,000 and 500,000 years ago.
