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The Geological, Isotopic, Botanical, Invertebrate, and Lower Vertebrate Surroundings of Ardipithecus ramidus

Giday WoldeGabriel et al.

Ardipithecus ramidus was found in exposed sediments flanking the Awash River, Ethiopia. The local geology and associated fossils provide critical information about its age and habitat.

Most of Africa's surface is nondepositional and/or covered by forests. This explains why so many discoveries related to early hominid evolution have been made within eastern Africa's relatively dry, narrow, active rift system. Here the Arabian and African tectonic plates have been pulling apart for millions of years, and lakes and rivers have accumulated variably fossil-rich sediments in the Afar Triangle, which lies at the intersection of the Red Sea, Gulf of Aden, and Main Ethiopian Rifts (see map). Some of these deposits were subsequently uplifted by the rift tectonics and are now eroding. In addition, volcanoes associated with this rifting have left many widespread deposits that we can use to determine the age of these fossils using modern radioisotopic methods.

Several of the most important hominid fossils have been found near the Afar's western margin, north and west of the Awash River (star on map), including Hadar (the “Lucy” site), Gona [known for the world's oldest stone tools at 2.6 million years ago (Ma)], and the Middle Awash (including Aramis). Cumulatively, these and nearby study areas in Ethiopia have provided an unparalleled record of hominid evolution.

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Map showing the Middle Awash area (star) and rift locations (red lines). Photo shows the 4.4-Ma volcanic marker horizon (yellow bed) atop the locality where the skeleton and holotype teeth of Ar. ramidus were discovered. Also shown are some of the fossil seeds.

Fossil-bearing rocks in the Middle Awash are intermittently exposed and measure more than 1 km in thickness. Volcanic rocks near the base of this regional succession are dated to more than 6 Ma. Its uppermost sediments document the appearance of anatomically near-modern humans 155,000 years ago. As is the case for many river and lake deposits, fossil accumulation rates here have been highly variable, and the distribution and preservation of the fossils are uneven. Alterations of the fossils caused by erosion and other factors further complicate interpretation of past environments. To meet this challenge, beginning in 1981, our research team of more than 70 scientists has collected 2000 geological samples, thousands of lithic artifacts (e.g., stone tools), and tens of thousands of plant and animal fossils. The emergent picture developed from the many Middle Awash rock units and their contents represents a series of snapshots taken through time, rather than a continuous record of deposition.

Ar. ramidus was recovered from one such geological unit, 3 to 6 m thick, centered within the study area. Here, the Aramis and adjacent drainage basins expose a total thickness of 300 m of sediments largely deposited in rivers and lakes, and on floodplains, between ~5.5 and 3.8 Ma. Within this succession, the Ar. ramidus–bearing rock unit comprises silt and clay beds deposited on a floodplain. It is bracketed between two key volcanic markers, each dated to 4.4 Ma. Their similar ages and sedimentology imply that the fossils themselves date to 4.4 Ma and were all deposited within a relatively narrow time interval lasting anywhere from 100 to 10,000 years. Today the unit is exposed across a 9-km arc that represents a fortuitous transect through the ancient landscape. The western exposure, in particular, preserves a rich assemblage of plant and animal fossils and ancient soils.

Fossilized wood, seeds, and phytoliths (hard silica parts from plants) confirm the presence of hackberry, fig, and palm trees. There is no evidence of a humid closed-canopy tropical rainforest, nor of the subdesertic vegetation that characterizes the area today. Invertebrate fossils are abundant and include insect larvae, broodballs, and nests of dung beetles; diverse gastropods; and millipedes. The terrestrial gastropods best match those seen in modern groundwater forests such as the Kibwezi in Kenya. Aquatic lower vertebrates are relatively rare and probably arrived episodically during flooding of a river distal to the Aramis area. The most abundant fish is catfish, probably introduced during overbank flooding and/or by predatory birds roosting in local trees.

Our combined evidence indicates that Ar. ramidus did not live in the open savanna that was once envisioned to be the predominant habitat of the earliest hominids, but rather in an environment that was humid and cooler than it is today, containing habitats ranging from woodland to forest patches.

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