The measure in the mud

At the GaJi10 site on the shore of Lake Turkana, 21 footprints dated to about 1.43 million years ago belong to eight individuals. The measurements, led by Kevin Hatala of Chatham University and the Max Planck Institute for Evolutionary Anthropology, point to stature up to 1.8 metres and a mass near 75 kilograms. That is the range of Homo erectus. The difference in body size we assumed separated the two lineages is not there, at least not in these prints.[1]

That occupies no small place in the story we tell. One lineage was narrated as heading towards us, the other as a heavy-jawed, stocky dead end, and the difference in height was the most easily pictured part of that account. Removing the difference does not remove the distinction in diet or jaw. What it removes is a shortcut the eye builds instantly. This is an interpretation; what was measured is the dimensions of the prints.[1]

The paper itself keeps that line. The size of the prints is a measurement; the reading that the eight walkers were mostly adult males travelling together without females or children is an inference drawn from print dimensions, not resting on skeletal remains. Every sentence about social structure comes off that second shelf and needs to be read that way.[1]

What it changes and what it does not

If eight adult males really walked together, it implies a social order in which tolerance and mate competition sit side by side, and that is knowledge no bone yields. But one set of tracks is one morning. A single-sex group may have gathered for a task, may be a chance aggregation, or may simply be the result of which prints survived. What the evidence holds is a scene, not a portrait of a species.[1]

What would move things forward is therefore simple. Further trackways from other sites and other periods measured the same way; and whether independent skeletal mass estimates for Paranthropus boisei agree with the range these prints give. Both can be published, and both may or may not turn one morning into a pattern.[1]

Today's gain is narrow. A boundary we drew with body size has lost one of its sides, and the questions we can ask about that relative have changed shape. We have not learned their society. A million and a half years ago, eight walkers crossed the shore of a lake; their height was close to ours, and knowing that makes us think again about which box we put them in.[1]