Research press release





今回Robyn Pickeringたちの研究グループが解析対象にしたのは、化石が出土した堆積層を取り囲む分厚いフローストーン(流華石)で、そこから採取した微量の放射性同位元素を測定することで年代を決定できる。鍾乳石や石筍などのフローストーンは、洞窟内を流れる水に溶けていた鉱物が沈殿して形成される。Pickeringたちは、320万~130万年前の6つの期間中にフローストーンが形成されたことを明らかにした。また、Pickeringたちは、これらの期間が湿潤期であり、フローストーンの形成をもたらす流水が多くなって洞窟が閉鎖状態になり、外部からの堆積物やヒト族の遺骸が流入できなくなったために、フローストーンの形成が途切れなく続いた一方で、化石記録に空白が生じたという考えを示している。


The fossil record of early hominins in South Africa is biased towards periods of drier climate, suggests a study of cave deposits published online this week in Nature. This finding suggests there might be gaps in the fossil record, potentially obscuring evolutionary patterns and affecting our understanding of both the habitats and dietary behaviours of early hominins in this region.

South Africa’s highest concentration of early hominin fossils comes from caves northwest of Johannesburg, in the so-called ‘Cradle of Humankind’. However, it has proven difficult to precisely date these fossils and assess their evolutionary history, owing to the disordered nature of the collapsed cave sediments in which the remains were preserved.

Robyn Pickering and colleagues instead analysed the thick flowstones that surround the fossil-bearing sediments, which can be dated by measuring their trace radioactive isotopes. Flowstones, such as stalactites and stalagmites, form in caverns when flowing waters deposit dissolved minerals. The authors find that flowstones were deposited in six intervals between 3.2 and 1.3 million years ago. These, they suggest, represent wetter periods, in which there was more water to deposit the flowstones and the caves were more likely to be closed off to incoming sediments and hominin remains, allowing flowstones to form uninterrupted, but leaving gaps in the fossil record.

When the climate became drier, however, vegetation cover would have diminished, increasing surface erosion and opening the caves to outside sediments and the preservation of hominin remains. Although the wet periods may have left gaps in the fossil record, the authors note that the flowstones nevertheless offer valuable insights into past changes in the climate.

doi: 10.1038/s41586-018-0711-0

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