Taphonomy of hominid sites, or what geology can tell us about our origins

Since a French geology student visited the remote region nearly 50 years ago, and brought back tales of fossil rich sediments, the area around the river Awash is considered a "must seen" for paleoanthropologists and geologist interested in the natural history of Africa.

Fig.1. Mentioned hominid bearing sites, Aramis near the river Awash in Ethiopia and Malapa in South Africa.

The presentation to the public of Ardipithecus ramidus, found at the site of Aramis in the catchment area of the river Awash, in October 2009 was a media event comparable to the presentation last week of Australopithecus sediba from South Africa.
But both described species are only a part of the recuperated evidence (in case of A. ramidus nearly 150.000 bone fragments), behind A. ramidus there lie 15 years of research, behind A. sediba 2 years - the paleontological and geological results of both sites shamefully received little attention in the mass media.
But the careful collection and examination of animal and plant fragments, and the geological framework, was rarely well documented as by these two discoveries.

The proposal of Darwin that Africa is the cradle of humankind led to the idea that the last common ancestor, which we share with the great apes, lived like the recent chimpanzees or gorillas in a forest habitat. Then, in 1925, the discovery of the first Australopithecus species, considered a early hominid, by Raymond Dart seemed to give further clues of human evolution.
The associated faunal remains showed that Australopithecus lived in a grassland environment, it was therefore speculated that the open grasslands of Africa - developing in the Pliocene ice ages - were exploited by early hominids and were therefore somehow integrally involved with the origins of upright walking, a possible key factor of our further evolution.

But the sediments and the paleontological content of the Lower Aramis Member (Sagantole Formation) at Aramis provided evidence that Ar. ramidus lived in a predominantly woodland setting - upright walking of early hominids was therefore primarily not an adaption to overlook or to colonize a grassy savanna.

Taphonomic assemblages often represent a collection of a variety of animals of different geographically locations and time periods. Carcasses of animals from different environments can be washed together, or natural traps, like caves, act as sample bag for many centuries, providing a false species assemblage. Accurate interpretation of fossil assemblages can be challenging. In the case of Aramis however, the sampled stratigraphic unit is sandwiched between two volcanic horizons, which yielded approximately the same age (4.4Ma), supporting the idea that the fossils represent a short time intervall.
The fossils of Aramis comprise a large variety of plants and animals, including insects, molluscs and bones of owls, parrots, porcupine, hyenas, bears, elephants, ancient horses, giraffes, antelopes and rhino.
The recovered bones of birds and mammals came from species living in, or associated with, closed forest or shrublands. One of the common larger mammals found associated with Ardipithecus is the spiral-horned antelope, or kudu (Tragelaphus).

Fig.2. A not so scientific reconstruction of a possible hominid habitat.

Today, these antelopes are browsers eating mostly Leaves, and they prefer bushy to wooded habitats. In contrast, remains of grazing antelopes are rare in the Aramis assemblage.
The environment of these animals can also be reconstructed by analyzing their teeth's. Carbon isotopes from tooth enamel yield dietary information because different isotope signatures reflect different photosynthetic pathways of plants consumed during enamel development. Therefore, animals that feed on tropical open-environment grasses (or on grass-eating animals) have different isotopic compositions from those feeding on browse, seeds, or fruit from shrubs or trees. The isotopic pattern of Ardipithecus is also similar to that of Tragelaphus, indicating little dietary intake of grass, and supporting the reconstruction that the animal lived predominately in the forest.

Additionally, oxygen isotopes, found also in the molecular structure of the enamel, can be used to reconstruct the relative humidity and evaporation (temperature) in the environment where the animal, and with it the teeth grow.

Fig.3. after WHITE et al.2009. Isotopic signature of fossil enamel from the Aramis Member.

The death of an animal is the last act in life, and the first step to go lost forever, or in rare cases become buried deep within earth, get fossilized and in even more rare cases being excavated by naked monkeys. But how to reconstruct these events, where no living eyewitnesses are allowed?

Let's see what the rocks can tell us. The two skeletons of Australopithecus sediba were discovered in cave infillings of the karst landscape of South Africa, in a massive, up to 1.5-m-thick stratigraphic unit containing abundant, well-preserved macro- and micromammal fossils, including articulated remains of Equus sp.
The poorly sorted, coarse-grained and cemented sandstone consists of grains with diameters ranging from 0.5 to 2.5 mm of quartz, chert, dolomite, peloids and, less commonly, iron oxide-coated grains, ooids, shale, and feldspar. Angular limestone blocks (smaller than 50 cm) and flowstone fragments (smaller than5 cm) occur throughout this facies. The heterogenic lithological composition tells us that the sediment - or parts of it- is allochtonous, the material of this facies was transported, maybe from outside, and deposited in the cave.

Fig.4. Geological map of the Malapa site, after DIRKS et al. 2010. The fossils of hominids where interbedded in facies D. The underlying flowstone was also dated by U-Pb on an age of ca. 2 Ma.

The heterogeny in the grains, ranging from sand to pebbles to larger boulders and fossils, and lacking sedimentary structures (like stratification) suggest the deposition of the unit as a single event, like a debris flow, maybe caused by a flood or a storm. The superb preservation and state of articulation of fossil material also indicate rapid deposition, limited transport distance, and laminar flow conditions consistent with debris flows.

These new data and the combination of different scientific approaches questions old certainties. The case of Ardipithecus suggests that the anatomy and behaviour of early hominids did not evolve in response to open savanna or mosaic settings.
In the case of A. sediba the geology tell us how hominids become fossilised and where we must search for them. The fossils found with A. sediba helped also to date the new species, and confirmed the radiometric ages, a faunal analysis is still missing, but who knows if further investigations will not force us again to change our understanding how we evolved.

REFERENCES:
BERGER et al. (2010): Australopithecus sediba: A New Species of Homo-Like Australopith from South Africa. Science, 328: 195-204
DIRKS et al. (2010): Geological Setting and Age of Australopithecus sediba from Southern Africa. Science, 328: 205-208
LOUCHART et al. (2009): Taphonomic, Avian, and Small-Vertebrate Indicators of Ardipithecus ramidus Habitat. Science 326: 66-66e4: DOI 10.1126/science.1175823
WHITE et al. (2009): Macrovertebrate Paleontology and the Pliocene Habitat of Ardipithecus ramidus. Science 326: 67-93; DOI 10.1126/science.1175822
WOLDEGABRIEL et al. (2009): The Geological, Isotopic, Botanical, Invertebrate, and Lower Vertebrate Surroundings of Ardipithecus ramidus. Science 326: 65-65e5; DOI 10.1126/science.1175817