<i>Sabretooth cats threatened most human ancestor</i>
Humankind's oldest known ancestor probably lived in fear of several large sabretooth cats that roamed the same ancient lakeside habitat in Africa.
Palaeontologists have identified two new sabretooth species among fossils unearthed at Toros Menalla in Chad.
In 2001, a team unearthed remains of a seven million-year-old human-like creature - or hominid - known as "Toumai" at the central African site.
Its discoverers argue that Toumai is the oldest hominid known to science.
The fossilised skull of Toumai (which means "hope of life" in the local Dazaga language of Chad) was found in the Djurab desert by a team led by Michael Brunet of the University of Poitiers, France.
The position of a hole at the bottom of the skull called the foramen magnum suggests that Toumai (Sahelanthropus tchadensis) walked upright - an important signature of the human lineage.
The brainstem enters and exits the skull through this hole; in great apes, it is positioned more towards the back of the skull. But in hominids - including Toumai - it is placed more towards the front of the skull.
The ancient fossil caused a worldwide sensation when it was unveiled in the pages of Nature journal in 2002.
However, the interpretation of Toumai as a human relative is controversial. The skull was distorted and, if any other parts of the skeleton happen to exist, none has yet been published in the scientific literature. It is also older than the date when genetics says that the human and chimp lineages diverged.
Nevertheless, palaeontologists have been busy studying the abundant fossil material unearthed at the site, steadily building a picture of the environment in which Sahelanthropus eked out its existence.
In Late Miocene times, this area of Chad must have had a lake, because palaeontologists have found the fossilised remains of fish, amphibians and crocodiles.
But they have also found evidence of grasslands, gallery forest and a desert.
Researchers have discovered the fossilised remains of a wide variety of carnivorous mammals at Toros Menalla.