Our knowledge of human evolution has always been limited by the meagre quantities of hominid fossils that have been discovered. But a system of limestone caves at Atapuerca in northern Spain has yielded an embarrassment of riches by comparison with an otherwise patchy hominid record.
The pit of bones
Since the 1980s, archaeologists have recovered the remains of 32 individuals from a chamber at the bottom of a 14 metre (45 foot) shaft known as La Sima de los Huesos ('The Pit of Bones'). The bones, which date to around 300,000 years ago, comprise 75% of hominid fossils known between 100,000 and 1.5 million years ago.
"Atapuerca was a good place to live. There was a river nearby and it was high up, so it was a good vantage point for hunters. The cave shelters there provided them with refuge," says Professor José Bermúdez de Castro of the National Museum of Natural Sciences in Madrid and co-director of the Atapuerca research team.
The remains at La Sima belong to a species of hominid called Homo heidelbergensis. But another site at Atapuerca has produced the remains of the oldest human ever found in Europe - a partial skull belonging to a young male who lived 780,000 years ago. This skull was discovered in 1994, when the Atapuerca team were excavating the site of an old railway cutting at the Atapuercan locality of Trinchera Dolina.
The specimen shares many similarities with Homo ergaster. But Professor Juan Luis Arsuaga of the Complutense University of Madrid and co-director of the Atapuerca research considers it different enough to give it a new species name: Homo antecessor. Not all palaeoanthropologists accept this classification because it is based on a juvenile specimen and key characteristics of a species often develop only in adulthood.
Professor Eudald Carbonell of the Rovira i Virgili University in Tarragona, Spain, says recent examination of the Trinchera Dolina remains suggests Homo antecessor could not have been ancestral to heidelbergensis. Instead, says Carbonell, antecessor was probably extinct by around 600,000 years ago.
Who were the first Britons?
By 500,000 years ago, another group of humans had reached Boxgrove in West Sussex, England. Although Boxgrove is not the site of the earliest human occupation in Britain (that gong goes to a 700,000-year old site in East Anglia discovered in June 2002), it is almost certainly the richest.
Here in 1993, archaeologists unearthed the shinbone of a heavily built male. The shinbone measured 35 centimetres (13 inches) long and had deep muscle markings, suggesting its owner stood around 180 centimetres (6 feet) and weighed 88 kilogrammes (196 pounds).
This individual belonged to Homo heidelbergensis. Skulls from elsewhere in Europe and in Africa show that heidelbergensis was developing a large brain, and the species is now seen as a key evolutionary link between ergaster and modern humans.
Today, Boxgrove is a gravel quarry. But half a million years ago, there was a beach and limestone cliffs here, with a tidal lagoon tucked behind a headland. Horses, megaloceros (giant deer), rhinoceros, voles and wolves occupied the landscape, along with a resourceful group of early humans.
Axes and spears
Bones from large animals such as rhinos, horses and hippos were covered with cut marks where Boxgrove man used stone blades to slash and butcher the animals for their meat. Crucially, the cut marks were found beneath the tooth marks of carnivores, indicating that humans got there before the scavengers. To archaeologist Mark Roberts, who led the Boxgrove excavation, this implies the Boxgrove people were hunting, not scavenging.
"Each (carcass) would have weighed 675 kilogrammes (1,500 pounds), a magnet for other predators. Yet each carcass was skilfully cut up. Fillet steaks were sliced from the spine and the bones were smashed to get out the marrow. Only hunters who were in total command of their patch could have done that," says Roberts.
Archaeologists have unearthed tens of thousands of flint and bone tools from Boxgrove. The site was effectively a tool factory. But the manner in which so many intact tools were abandoned may open an intriguing, yet perplexing window into the minds of heidelbergensis.
Evidence of ritual?
"They (heidelbergensis) make these handaxes that to our eyes look perfectly serviceable. And yet, they've tossed them away. There's a lot of making things but not actually using them," says Professor Clive Gamble of the Centre for the Archaeology of Human Origins at the University of Southampton, England.
Gamble cites wooden spears found preserved in a bog at Schöningen, Germany, and are associated with horse bones. Dated to 400,000 years ago, the spears provide the first hard evidence of human hunting and are weighted at the ends to be thrown like a javelin.
"I just wonder whether the Schöningen spears were ever used. Yes, there are horses at the site, but are the tips of the spears damaged? You'd think spears like that would break after they'd been jammed in a few horses," muses Gamble. For heidelbergensis, tools and hunting weapons may have played an important role in social display, one that we don't yet fully understand and may even border on ritual.
"They may have been more interested in making things as a demonstration of who they were and what was important to them. Killing horses was probably something they did once a week," Gamble remarks.
In 2003, the Atapuerca team announced the discovery of a single stone handaxe found buried amongst the human remains in the Pit of Bones. According to the researchers, its strange colour may mark it out as evidence of the first funeral rite, which suggests the hominids at Atapuerca were deposited in the pit deliberately.
The rich assemblage of human remains from Atapuerca offers a fascinating but more disturbing insight into the life - and possibly the mind - of heidelbergensis. Since the cache of bones was first described in 1997, archaeologists have puzzled at how the remains of 32 individuals accumulated at the bottom of this narrow shaft.
The Atapuerca researchers conclude that the remains come from one group and were dumped there in the space of a year. Almost all are adolescents, with the exception of two adults and a child.
"It must have been a catastrophe. It could have been due to an epidemic, or various illnesses," remarks Bermúdez de Castro. Many bones display signs of poor health, including malnourishment, infections and abnormal growths.
But Carbonell thinks he can explain the large numbers of adolescents in the pit. "In my opinion, the death of many juveniles is very normal for populations living the Middle and Late Pleistocene periods," he explains. "Females of 16-18 years may have died in childbirth. Males aged 9-12 years are at a stage where they leave the protection of their families but are still fragile in the face of nature. They are more likely to have been bitten or killed by wild animals," adds Carbonell.
However, Professor Peter Andrews of the Natural History Museum in London thinks that the bodies could have been sludged into the pit through a mudflow. "One has to put some caution into this because it has been suggested that this is a secondary deposit and therefore could be accidental," says Professor Chris Stringer, also of the Natural History Museum.
If the bodies were dropped into the pit as part of a burial ritual, Atapuerca could provide the first clear evidence of symbolic thinking in an early hominid. "It is very hard to get colleagues to accept evidence of ritual for early humans," says Bermúdez de Castro.
These narrow glimpses into the mind and everyday life of heidelbergensis are at once fascinating and frustrating. But this early human was undoubtedly developing a complex mind. Once this boundary had been reached, there was no turning back.