Shum Laka

Why this is extraordinary

Shum Laka is a rockshelter in the Grassfields of western Cameroon, about 15 kilometres from Bamenda, on the inner wall of the Bafochu Mbu caldera. People used it from about 30,000 calendar years before present. The layers that matter for this room sit between the end of the Later Stone Age, about 8,000 years ago, and the beginning of the Iron Age, about 2,500. In that span, sometimes called the Stone to Metal Age, new stone tools and pottery come in slowly. Food remains are still mostly from foraging, with more and more of the oily fruit Canarium schweinfurthii.

Eighteen human skeletons were found, in two burial phases. The early phase, in the ochre ashy layer, has four people: two adults in single graves and two children in one double grave. The later phase, in the grey ashy layer, has at least fourteen, children and adults. Genomes were read from four of the children. In the double grave a boy of about 4 lay on the legs of an adolescent of about 15. Their calibrated ranges, from the same temporal bones that gave the DNA, are 7,920 to 7,690 and 7,970 to 7,800. The later pair, buried side by side, are a boy of about 8 and a girl of about 4, at 3,160 to 2,970 and 3,210 to 3,000. Collagen quality was good. Carbon-to-nitrogen ratios sat between 3.3 and 3.4.

The surprise is who they are related to. The paper’s own comparison puts all four closest to hunter-gatherers of western Central Africa today. The same paper’s mix is more divided than that comparison carries alone. In its model, the four children can be read as about 35 percent ancestry related to hunter-gatherers from western Central Africa and about 65 percent from a basal West African source. The same paper argues for a further deep source, which it calls a ghost, that contributed a minority of the ancestry in West Africans and in East African hunter-gatherers, and not the bulk of these four children. People now living in western Cameroon, and Bantu-speaking populations across the southern half of the continent, are not, on the paper’s argument, substantially descended from them. That is awkward on a map, because the Nigeria-Cameroon borderland is the probable homeland of the Bantu languages, and the spread of those languages is placed between about 3,000 and 1,500 years ago. One adolescent carries Y chromosome haplogroup A00, the oldest known branch of the modern human Y tree, splitting about 300,000 to 200,000 years ago from every other known lineage, and today almost only found in this same region, among the Mbo and the Bangwa. The branch survived here. The rest of the genome did not, on this argument, become the later farmers. Four genomes remain the sample.

The surprise at this shelter is who the buried people are related to, not which way the mouth faces. No solstice orientation of the graves, and no star, is published in the sources used here. A rockshelter can open toward a valley for the light and the weather without anyone marking a sunrise. Until a bearing is printed, the shelter is a place that kept bones through a long span. It is not an observatory. The ancestry already argued here does not point at the sky either.