South African Deputy President Kgalema Motlanthe, center, and professor… (Alexander Joe / AFP/Getty…)
The middle-aged woman and the young boy, perhaps her son or simply another member of her tribe, were out hunting on the African plains or maybe looking for water in the midst of a drought when they fell into a sinkhole, dying almost instantly.
Shortly thereafter, a monsoon or a flood washed them into a deeper basin, where they were covered with mud and rapidly fossilized.
In 2008, nearly 2 million years later, another boy, 9-year-old Matthew Berger, discovered part of their skeletons outside the Malapa cave north of Johannesburg, South Africa, a find that experts have dubbed one of the most important of recent times.
Matthew was accompanying his father, paleoanthropologist Lee R. Berger of the University of the Witwatersrand in Johannesburg, who had used images from Google Earth to identify caves in the Cradle of Humankind World Heritage Site that might hold fossil deposits.
On his first visit to the Malapa cave, Berger took Matthew and suggested that the boy look around outside for fossils. Within 15 minutes, the boy returned carrying a block of stone bearing a hominid collarbone and a jawbone that had apparently been thrown out of the cave by miners.
Excited by Matthew's discovery, a team led by Berger began excavating the cave and found the rest of that skeleton, which appears to be that of an 11-to-12-year-old boy, and the skeleton of a woman in her mid-30s.
The hominid pair may be direct ancestors of humans or they may be from a closely related branch on the human evolutionary tree, South African researchers reported Thursday in the journal Science.
Either way, experts agree that the rare discovery of nearly intact skeletons provides a look at evolution during what is considered one of the most significant periods of human development, when hominids were changing from the ape-like genus known as Australopithecus into the more modern forms we now know as Homo.
The skeletons are the only complete specimens that fall between the Australopithecus afarensis known as Lucy, dating from 3 million years ago, and the Homo erectus known as Turkana boy, dating from 1.5 million years ago.
"This give us a good chance to look at the rates of change of various parts" of the body, said biological anthropologist William Kimbel of Arizona State University, who was not involved in the discovery. It is exceptionally rare to get such insights from single individuals in which all the body parts are together, he said.
"There are very few things that we can call a skeleton from that time period," said biological anthropologist Christopher Ruff of Johns Hopkins University School of Medicine, who was also not involved in the research. Most often, researchers discover isolated craniums, jawbones or limbs and must extrapolate what the species looked like.
The research process is very much like the ancient tale of the seven blind men who each touch a different part of an elephant and draw widely differing conclusions about the beast's shape, Ruff said. The new specimens are important because they provide a look at the whole elephant.
In addition to the hominid skeletons, the team found fossilized remains of at least 25 animal species, including antelope, mice, saber-toothed cats, a wildcat, a brown hyena, a wild dog and a horse.
They also found at least two other hominid skeletons, another woman and an infant, Berger said in a news conference, but they are not yet reporting on those.
All of the skeletons are in excellent shape because their fall into the pit protected them from scavengers and they were all fossilized rapidly.
The two "really remarkable skeletons" of primitive hominids that the team is reporting were quite surprising when they were completely unearthed, showing some characteristics of primitive australopithecines and some of the more modern Homo, Berger said.
The boy has a brain volume of about 420 cubic centimeters, less than the 510 cubic centimeters of the smallest known example of Homo and much less than the 1,200 to 1,600 cubic centimeters of modern humans.
The woman and the boy were about 4 feet tall, about the same as australopithecines. They also had long arms like those of the australopithecines. The woman weighed about 70 pounds and the boy about 60.
But the skeletons also had the smaller teeth, more prominent nose and less pronounced cheekbones of Homo erectus, as well as longer legs and changes in the pelvis characteristic of more modern hominids.
This combination of traits has never been observed in a single specimen. Harking back to the story of the blind men and the elephant, if anthropologists had only the teeth, they would think they had a species of Homo; if they had only the cranium, they would think it was Australopithecus; and if they had only the arm bone, they would think it's an ape, Berger said.
"What we now know from this is that you need a lot more than just one part of the [body] to define a genus," he said.
The lower limbs and the pelvis of the creatures, which the team has christened Australopithecus sediba, indicate that they easily walked upright. ("Australopithecus" means southern ape, and "sediba" is Sotho for natural spring, fountain or wellspring.) The long arms, however, indicate that they still climbed trees. Despite the name, then, "this is very different from australopithecines," Berger said.
There are three primary possibilities, Berger said: Australopithecus sediba is either a direct ancestor of Homo erectus, an ancestor of some of the variants within the Homo lineage, or "a very close side branch mimicking the earliest members of Homo."
Berger said the team would hold a competition among African children to give the boy a common name.