Africa is, of course, the original homeland of our species, Homo sapiens, and is the place where numerous other earlier human species, such as Homo habilis, Homo rudolfensis, and Homo ergaster originated. The most recent addition to the human family, in terms of paleoanthropological finds, is Homo naledi. For paleoanthropologists and others interested in human ancestry, Homo naledi presents an interesting problem: we don’t know how old it is.
The bones which are now designated as Homo naledi were found in the Rising Star Cave in South Africa. The skeleton suggests that this is an ancient species with a blend of human (i.e. Homo) and australopithecine features. The skull is relatively small—560 cc for males and 465 cc for females—but has a shape that is more human than australopithecine. Homo habilis, which is often seen as the first human species, had an average brain size of 650 cc and dates to 2.4 to 1.5 million years ago. If we assume that increasing brain size is one of the primary features of human evolution, then Homo naledi may be earlier than Homo habilis. Anatomically modern humans, by the way, have an average brain size around 1,350 cc.
The human hand, which is very different from the hand of an ape, made it possible for humans to craft a variety of tools from different materials. In Homo naledi, the researchers have noticed that the wrist, palm, and thumb are human-like which may have allowed for tool use, but fingers are curved like those of australopithecines and appear to be adapted to climbing.
The pelvis of Homo naledi is flared outward, another similarity with the australopithecines. Conversely, it has feet which are very human-like.
Anatomically, Homo naledi looks like it should be transitional between the austrolopithecines and Homo and this is should be dated to somewhere between 2 and 2.5 million years ago. If, however, it is found to be earlier than this it will challenge the idea that Australopithecus afarensis (that’s Lucy’s species) was in our direct evolutionary lineage. On the other hand, if it is found to be more recent, say less than a million years, it would have shared the African landscape with larger brained humans.
Dating ancient fossils in Africa is generally done by dating the stratigraphy in which they are found. Frequently in Africa there are layers of volcanic ash above or below the fossils which can be accurately dated from the clocklike decay of the radioactive elements within the ash. If the geological material below the fossils can be dated, then we know the fossils must be younger. Similarly, if the material above the fossils can be dated, then we know the fossils are older. In the Rising Star Cave, where the bones of Homo naledi were found, however, there is no stratigraphy: the bones were simply on the cave floor.
For archaeological sites more than 100,000 years old, archaeologists are often able to use potassium-argon dating. This is a technique used by geologists to date rocks which are as much as 2 billion years old to as little as 50,000 years old. This is an important dating method for archaeologists who are exploring early human evolution and human origins. In their book Making Silent Stones Speak: Human Evolution and the Dawn of Technology, Kathy Schick and Nicholas Toth report:
“One of the most useful approaches to dating African sites has been the potassium-argon dating method.”
Volcanic material contains both potassium (K) and an argon isotope (Ar-40). At the time of formation, all of the argon is released, but the potassium still present has a radioactive isotope (K-40) which becomes a part of the inert argon isotope Ar-40 as it decays. The process of decay takes place at a known rate and therefore it is possible to date the material. In an entry on potassium-argon dating in
The Oxford Companion to Archaeology, Brian Fagan reports:
“Because many archaeological sites were occupied during a period when extensive volcanic activity occurred, especially in East Africa, it is possible to date them by associations of lava with human settlements.”
Potassium-argon dates can be taken only from volcanic rocks. Concerning the modern use of potassium-argon dating, Brian Fagan writes:
“In recent years, computerized argon laser fusion has become the technique of choice. By steering a laser beam over a single irradiated grain of volcanic ash, a potassium-argon specialist can date a lake bed layer, and even a small scatter of tools and animal bones left by an early hominid.”
The grain of volcanic ash becomes white hot and gives up a gas. This gas is then purified, charged with an electron beam, and hurled by a strong magnet against a device which counts its argon atoms. Fagan reports:
“By measuring the relative amounts of the two isotopes of the element, researchers can calculate the amount of time that has elapsed since the lava cooled and the crystals formed.”
Fission track dating can be used to date materials such as volcanic rocks, and crystalline materials from 20 years to 1 billion years. In their book
The Dawn of Human Culture, Richard Klein with Blake Edgar write:
“The fission-track method is a cousin of potassium/argon dating that depends on the radioactive decay of naturally occurring uranium within ancient volcanic rocks or tektites, and like the potassium/argon method it estimates the last time the rocks were heated to a very high temperature.”
In their
A Dictionary of Archaeology, Ian Shaw and Robert Jameson report:
“In archaeology the technique has largely been used for dating volcanic glass and minerals (usually zircon) associated with deposits containing hominid remains.”
Dating in archaeology, and particularly in the ancient African sites, often uses a variety of techniques, such as those described above. It is not uncommon for dates to be carefully examined and debated for archaeology is a science and therefore debate, discussion, and disagreement are encouraged.