But even the existing fossil record can still surprise us. Fifty years after Darwin published his cautious prediction that Africa would turn out to be the continent of our origin, the Broken Hill skull was fortuitously discovered, and so began the process of proving he was right. But the skull was not immediately recognized for what it was, and might easily have been lost to science. On 17 June 1921, miners quarrying a small hill full of metal ore near the town of Broken Hill (now Kabwe), in what was then Northern Rhodesia (now Zambia), uncovered a skull coated in brown sediment. Its huge staring eye sockets apparently scared them so much that they all ran off. Their supervisor, a Swiss miner called Tom Zwigelaar, was somewhat braver and got someone to photograph him at the site of discovery holding the relic.
The Broken Hill cranium (“Rhodesian Man”) was the first important human fossil ever found in Africa, and even now it is one of the most impressive. It resides in a metal safe outside my room at the Natural History Museum in London and is one of the treasures of the Palaeontology Department—shiny brown and beautifully preserved, with massive brow ridges glowering over those empty eye sockets. It was first put into its own species—Homo rhodesiensis—by Arthur Smith Woodward of the museum in 1921 and has been named and renamed ever since. In 1930 it was described by the Czech-American anthropologist Aleš Hrdlička as “a comet of man’s prehistory” because of the difficulty of deciphering its age and affinities. Despite its completeness and apparent primitiveness, its exact place in human evolution still remains unclear, because it has never been properly dated. When I was a student, it was still being used as evidence that Africa was a backwater in human evolution, because such a primitive specimen was living there only 50,000 years ago, while much more advanced humans had evolved in Europe and Asia. Now, as I said, it is usually placed with fossils like Bodo and Elandsfontein as representing the African component of our ancestral species Homo heidelbergensis from about half a million years ago.
As a small boy, visiting with my parents, I can vividly remember seeing the Broken Hill skull (or rather a plaster cast of it) on display at the Natural History Museum and being intrigued by its primitiveness and mystery. Ever since, I have nurtured the hope that I could help to place it definitively in the story of human history, either as an ancestor or as a distinct species that had died out without contributing to our evolution. I studied it for my Ph.D. in 1971, and I have regularly included it in my analyses of fossils, making it a central part of my concept of Homo heidelbergensis as a species that represented the last common ancestor of modern humans and Neanderthals. But without knowing how old it was, its precise place in human evolution has remained elusive and seemingly beyond reach, given the complete destruction of the site from which it came. But at last this is proving possible—and the results are surprising me as much as anyone else. For at least fourteen years prior to the discovery of the skull, miners had been digging through a fifteen-meter-tall column of fossilized bones, and because these were heavily impregnated with mineral ores, they were throwing them all into the smelter—I’d rather not think about what might have been lost!
After its discovery, other fossil human and animal remains and artifacts were recovered in the site and around the locality by people like Aleš Hrdlička and Louis Leakey, including from the mining dumps and the miners’ huts, but only two human bones were found close to the skull, and at the same time. These were a long and straight shinbone (tibia) and the middle part of a thighbone. The latter find had a particularly interesting history. It was found by a Mrs. Whittington, who happened to be visiting her sister, whose husband worked at the mine. She was obviously an adventurous woman and was lowered down on a rope to collect it, but it was then virtually forgotten until Desmond Clark negotiated its transfer from a Rhodesian museum to London in 1963. Two other intriguing nonhuman finds also proved to be important in unraveling the lost history of the Broken Hill skull. One was a thin and mineralized yellowy brown silty deposit, which the miners collected because they mistakenly thought it was the mummified skin of Rhodesian Man. The other was a mass of tiny bones found around the skull and even cemented inside it. Originally thought to be those of bats, these actually represent the bones, jaws, and teeth of various small mammals, and they provide important information on the age of the skull and on where it originally lay in the cave.
The skull itself shows a strange combination of features. On the one hand, the brain size is only just below the modern average, but on the other, the face is big and the braincase shape decidedly primitive—long and low with enormous brows (monstrous was the word that Hrdlička used), with an angulated back to the skull and a transverse bony ridge reminiscent of Homo erectus. The cheekbones are not hollowed out as in modern humans (although a second upper jaw fragment found elsewhere in the site does show this feature), and the teeth are riddled with disease to an extent unusual for an early human: many are decayed and some of their roots are abscessed.
There are several other curious features, including a small and nearly circular hole in the left side of the braincase. Over the years, this has been suggested to be from a spear point, a lion’s canine, or even primitive surgery. But not long after I joined the Natural History Museum, I learned of an entirely new idea. A British newspaper was serializing a book called Secrets of the Lost Races and requested permission to print a picture of the Broken Hill skull. When I asked what caption would accompany the illustration, I was told that it would say that this was the skull of a Neanderthal shot by an alien’s bullet 100,000 years ago! I pointed out that the fossil wasn’t really that of a Neanderthal, that it was probably much older than 100,000 years, and that a bullet hole would probably have been accompanied by radiating cracks; I also asked, what self-respecting alien would be using something as primitive as bullets? Nevertheless, it was agreed that the newspaper could have its photo if it included the statement that recent research suggested the hole showed signs of healing and was probably caused by disease emanating from within the braincase. Of course such scientific data didn’t suit the paper’s agenda, and it included a drawing of the skull instead, leading me to suffer several frustrating weeks as members of the public telephoned, wrote, or even turned up unannounced at the museum asking to see “the Neanderthal shot by the spaceman”!
The tibia found with the skull represents a tall individual of about 180 centimeters, but for a presumed male heidelbergensis, he was probably not that heavy at around seventy-five kilograms, compared with estimates for the fossils from Boxgrove (tibia) and Bodo (skull) of over ninety kilograms. So Rhodesian Man probably had the tall and relatively slender build we might expect for an inhabitant of the drier tropics today, although more robust and muscular. The tools from the site are a varied bunch, and none can be closely associated with the skull except for a round stone found with the femur fragment and which, along with others from the site, has been interpreted as a projectile, a pounding stone, or even a bola (a hunting or herding weapon formerly used in South America, consisting of balls connected by string or rope and which are thrown to entangle the legs of the target animal). Other artifacts included flakes, scrapers, and even some possible bone tools, but none of these look very ancient, suggesting a maximum age of perhaps 300,000 years. While some of the animal bones found around the site may have been those of prey, no studies so far have found convincing evidence of butchery, and given the complete destruction of the site, it is impossible to tell whether the bones and stone tools come from inhabitants of the cave(s) in the mined hill or found their way in through some other means.
I have been working on trying to date the Broken Hill skull more precisely for about fifteen years, with a number of collaborating scientists and even mineralogist colleagues at the Natural History Museum. The main methods we have been using (see chapter 2) are ESR (electron spin resonance) on a tooth fragment from the skull and uranium-series dating from various bones and sediment from the site. It would normally take great care and courage to remove an enamel fragment from one of the pr
ecious teeth of the Broken Hill skull, but a previous accident worked in our favor in this case. Some unknown member of staff or a visitor had accidentally knocked the corner from one of the molars and, rather than reporting it, had simply glued the piece back on! When one of my eagle-eyed colleagues, Lorraine Cornish, spotted this, she dissolved the glue and we had the perfect fragment of enamel to date. But one of the unknowns in ESR dating is the degree of radiation received by the enamel fragment since it was buried, and this has to be reconstructed from site data, which in the case of Broken Hill was severely lacking, with the complete destruction of the original location by mining. However, chunks of sediment and bone breccia were saved from the mine, in some cases because they also contained interesting minerals, and others were collected after the discovery of the skull, so these could be measured to help reconstruct the burial environment.
One of the worst-case scenarios in ESR dating is underwater burial, since water interferes with the accumulation of the ESR signal. There was plenty of evidence from the mine records that the level where the skull had been found had to have water regularly pumped out as it actually lay below the existing water table. However, two other clues I already mentioned became critical here. First, the “skin” that the miners thought they had found was actually layered sediment impregnated with minerals, which must have been laid down relatively horizontally and could not have formed under water. Records made at the time of discovery stated that it lay near the skull and tibia, at a steep angle, suggesting that it had fallen from higher up. Second, we know that the skull was covered in, and even contained, many bones of small animals such as shrews, and the mine records clearly document layers of small mammal bones at a much higher level, far above the place where the skull was actually found. So it seems likely that quarrying at the base of the sediments led them to constantly collapse down, and the skull was almost certainly derived from the higher levels of the site, above the water level.
Now, when we factor everything together, the ESR signal in its tooth enamel suggests that the skull is actually between 200,000 and 300,000 years old. And two other age estimates from the femur fragment and the so-called skin suggest the real age could be closer to 200,000 rather than 300,000 years. It is possible that the skin accumulated above the level of the skull and femur before they all collapsed down, so it could be somewhat younger than them, but certainly there is nothing here to indicate that this assemblage is anything like 500,000 years old. Such a surprisingly young age is not contradicted by the artifacts known from the site, which have early Middle Stone Age affinities, nor from studies by the paleontologists Margaret Avery and Christiane Denys of the small mammal accumulations closely associated with the skull, which in species represented match those known from African sites like Twin Rivers, dated in the range 200,000 to 300,000 years old. If the Broken Hill skull, one of the best-preserved relics of Homo heidelbergensis, is actually less than 300,000 years old, what does this mean for our models of human evolution and for the origin of our species?
The result does have important implications for our reconstructions of recent human evolution because, as I explained, the Broken Hill fossil has been a cornerstone of the assumed gradual evolutionary sequence from archaic to modern humans in Africa. Dating Broken Hill to about 500,000 years placed it some 300,000 years older than the first known modern humans, allowing plenty of time for the necessary changes to happen. But the new dating makes Broken Hill only somewhat older than Omo 1 at about 195,000 years, and perhaps close in age to the more modern-looking Florisbad and Guomde fossils. This would imply either that there was a very rapid evolutionary transition to the earliest modern humans about 250,000 years ago, or that Africa contained great variation in its human populations at that time. Could that variation have even extended to the coexistence of different human species? We already discussed the puzzling variation between the two apparently contemporary Omo Kibish fossils 1 and 2, with skull 1 looking decidedly modern and skull 2 having a more primitive braincase, with an angled back, and we mentioned the rather archaic-looking rear of the Herto adult skull. Moreover, there are other primitive-looking fossils in Africa (such as those from Ngaloba and Eyasi in Tanzania) that overlap the dates we currently have for the oldest modern-looking humans, and I will discuss a particularly striking example next. All of this means that I am reconsidering many of my previous views on the origin of our species in Africa, and I now think we need to talk about origins, rather than a single point of origin.
I pointed out in the previous chapter that the nature of the manufacturers of Paleolithic tools from many parts of Africa remains completely unknown, since there are no associated fossils. This is especially true of artifacts from West Africa, where the oldest known fossil, from the Iwo Eleru rock shelter in Nigeria, is thought to be less than 15,000 years old. This poorly preserved skeleton was excavated from basal sediments at Iwo Eleru in 1965 by the archaeologist Thurstan Shaw and his team and was associated with Later Stone Age tools. That latter fact alone would indicate a relatively young age, and a radiocarbon date on a piece of charcoal suggested an age of about 13,000 years. The skeleton, and particularly the skull and jaw, was studied in 1971 by Don Brothwell, my predecessor at the Natural History Museum, and he argued that while the specimen could be related to recent populations in West Africa, it actually looked rather different from them. I studied the skull for my Ph.D., with surprising results. I also found that it did not closely resemble recent African populations, but in its long and low shape it was actually closer to early moderns such as those from Skhul, and even to more primitive specimens such as Omo 2. This was decidedly odd for such a young skeleton, and so I recently collaborated in a new study of the specimen with the archaeologist Philip Allsworth-Jones, the dating expert Rainer Grün, and the anthropologist Katerina Harvati. We first checked with Thurstan Shaw whether there were any hints that the skull could have been much older than previously suggested, and there were none. With the help of the Nigerian archaeologist Philip Oyelaran, I obtained a fragment of bone from the skeleton and passed it to Grün in order to check its age directly. His determination from a direct uranium-series age estimate is that the bone is unlikely to be older than 20,000 years, consistent with the stratigraphy and associated archaeology and radiocarbon date.
Finally, could Brothwell and I have been wrong about the unusual shape of the skull? Harvati used state-of-the-art geometric morphometric scanning techniques on an exact replica of the skull (which is now in Nigeria) and found, as we did, that it was quite distinct from recent African crania, and indeed from any modern specimen in her comparative sample. Her results placed the skull closest to late archaic African fossils such as Ngaloba, Jebel Irhoud, and Omo 2—all thought to be at least 140,000 years old. So what does this mean? Because of the poor preservation of Pleistocene bones in West Africa, we have no other data on the physical form of the inhabitants of the region during the whole of the Pleistocene, so we have to be careful in interpreting an isolated specimen such as Iwo Eleru. But it does not seem to be diseased or distorted, and does indeed seem to indicate that Africa contained archaic-looking people in some areas when, and even long after, the first modern-looking humans had appeared. Support for this view comes from the work of the anthropologist Isabelle Crevecoeur. Her restudy of the numerous Ishango fossils from the Congo showed that these Later Stone Age humans were similar to Iwo Eleru not only in age but also in the surprisingly archaic features found in their skulls, jaws, and skeletons.
African fossils Ngaloba (Laetoli H.18, Top) and Iwo Eleru (bottom). They resemble each other, despite Iwo Eleru being less than 20,000 years old, compared to Ngaloba’s 140,000-year-old archaic features.
Africa today has the greatest internal genetic variation of any inhabited continent, and its skull shapes show the highest variation. This is usually attributed to its greater size, larger ancient populations, and deepest time lines for humanity. But could those time lines go back even farther than we thought? Did the early modern
morphology evolve gradually and then spread outward from a region like East Africa, completely replacing archaic forms within Africa and then outside (as mtDNA data would suggest)? Or could there have been a version of assimilation or multiregional evolution within Africa, with modern genes, morphology, and behavior coalescing from partly isolated populations across the continent? Given its huge size, complex climates, and patchworks of environments, Africa could have secreted distinct human populations just as easily as the rest of the inhabited world. So was the origin of modern humans there characterized by long periods of fission and fusion between populations, rather than representing a sudden single event? And was the replacement of the preceding late archaic peoples not absolute, so that they were partly absorbed by the evolving moderns rather than completely dying out? In which case, did early Homo sapiens forms, and even the preceding species Homo heidelbergensis, survive alongside descendant modern humans?
This might explain archaic aspects in the shape of the Herto, Omo 2, and Iwo Eleru crania. In part they resemble archaic crania like Broken Hill, assigned to Homo heidelbergensis, so is this mosaic anatomy just a primitive retention from more ancient ancestors, or is it a sign of gene flow from contemporaneous African populations that still retained such features? My gut feeling is that some (but clearly not all) of the “ancient” DNA markers being picked up outside of Africa and used to argue for gene flow from non-African archaics will turn out to be traces of admixture that had actually happened in Africa. (A good example of this is the microcephalin gene discussed earlier.) Those traces were then carried from there in the modern human dispersal(s), followed by the operation of selection and drift on those populations, producing frequency changes in the genes when comparing the groups with each other and with their African counterparts. So while some archaic genes certainly were picked up by interbreeding outside of Africa, some were also acquired before the exodus, and yet others could even have been added in Africa, after it.
Lone Survivors Page 29