I’ve often discussed the origin of various elements and compounds on Earth – most notably the isotopic ratio of water, and what that might tell us about the origin of terrestrial water (1). Data about this can help provide evidence for the Earth’s early history, and often the data is inconsistent with the general theories of oceanic origin, like the ‘late veneer theory’, for instance, where the bulk of terrestrial waters were supposed to have been supplied by comets. It turns out that the water was on this planet all along (2,3), raising questions about why the Sun’s heat had not driven this relatively volatile resource away from the primordial Earth during the early history of the solar system.
Despite such evidence, the ‘late veneer theory’ continues to hold ground for many scientists, and tends to go unchallenged within the science media. This is apparent within the following excerpt about a new paper on the mysterious presence of a particular isotope of the noble gas xenon found in ancient terrestrial water encased in rock:
“The scientists have been analysing tiny samples of ancient air trapped in water bubbles found in the mineral, quartz, which dates back more than three billion years. The team found that the air in the rocks is partly made up of an extremely rare form of the chemical element, xenon. It is known as U-Xe and what makes it so rare is that it isn’t usually found on Earth. The component is not present in the Earth’s mantle, nor is it found in meteorites.
“Therefore, the team believe that the U-Xe must have been added to the Earth after a primordial atmosphere had developed. Simply put, comets are the best candidates for carrying the U-Xe to the planet. Co-author, Prof Ray Burgess, from Manchester’s School of Earth and Environmental Sciences explains: “The Earth formed too close to the Sun for volatile elements, such as U-Xe, to easily condense and they would have rapidly boiled off the surface and been lost to space.
“”The reason that oceans and an atmosphere exist at all is because volatiles were still being added after the Earth formed. The puzzle is in identifying where the volatiles came from and what objects carried them to the early Earth. The difficulty is that many of the different volatile ingredients that were originally added have been thoroughly mixed together by geological processes during Earth’s long geological history.”” (4)
It turns out that xenon, in general, is mostly absent from the Earth’s atmosphere, particularly compared to other noble gases like argon. No one knows why. Perhaps the missing xenon is encapsulated within rocks buried deep within the Earth. Or perhaps, conversely, it has been driven off the Earth because it is not easily captured by rocks like perovskite (5). Xenon is missing from Mars, too, which may allude to its propensity for loss from a weak atmosphere.
New species of archaic humans seem to pop up pretty frequently these days. If you accept the evolution by natural selection model, then the human lineage is less of a linear progression from primate ancestors, and more of a messy demolition derby of sub-species which came and went, branching out into dead-end alleys of development. Only one line survived the ravages of the last few hundred thousand years – us. The remains of the rest, the human species which didn’t make it and succumbed to extinction, like Homo floresiensis, are being dug out of caves around the world.
The latest of these discoveries are the Homo naledi hominins, who appear to have lived in southern Africa some 300,000 years ago around the same time that early humans were emerging as a species (1). The remains of these hominins was discovered in the complex Rising Star system of caves in South Africa a couple of years ago (2). The bones littered a pit-like chamber which was very difficult to access. The bones provide palaeontologists with a curious set of archaic specimens. The small skull size of Homo naledi, providing space for a brain just half the size of a modern human, indicated a primitive hominin.
The small brain size led the palaeontology team, led by the maverick academic Lee Berger, of Johannesburg’s University of the Witwatersrand, to conclude that the species had lived perhaps 2 – 3 million years ago. The shape of the skull was suggestive of early Homo species, including Homo erectus, Homo habilis or Homo rudolfensis. However, various aspects of the skeleton more closely resembled modern humans – their wrists, the feet, the lower part of the pelvis, some of their teeth (3). It’s a very odd mix indeed:
““You could almost draw a line through the hips—primitive above, modern below,” said Steve Churchill, a paleontologist from Duke University. “If you’d found the foot by itself, you’d think some Bushman had died.”” (2)