For many decades, a disparity between the resolution of long and continuous marine paleoclimate records versus fragmentary and time-averaged terrestrial records has hampered our ability to establish precise links between human evolution and major environmental changes. However, a recent proliferation of fieldwork, new drilling campaigns targeting highly detailed and relatively continuous paleolake records, and novel geochemical approaches are helping to assess terrestrial environmental dynamics at finer resolutions.
Key events in human evolution are thought to have occurred between 3 and 2.5 Ma, but the fossil record of this period is sparse. Here, Alemseged et al. Despite this progress, it remains the case that fauna, particularly hominins, are poorly sampled from the crucial time range between 3 and 2.5 Ma because fossiliferous sediments dating to this interval are rare.
The Mille-Logya Project (MLP) is a new paleoanthropological site, dated from ca. 2.9 to 2.4 Ma, at the northeast end of the well-known Plio-Pleistocene sites in the Awash Valley of the Afar Regional State, Ethiopia. Research at Mille-Logya started in 2012 and our team conducted systematic geological and paleontological surveys in 2014, 2015, 2016, 2018, and 2019.
Thus, they offer a unique opportunity to elucidate major events in human evolution including the transition from Australopithecus to Homo, the emergence of Paranthropus, and the advent of manufactured stone tools. Furthermore, although Australopithecus afarensis is the most abundantly preserved hominin from the region between 3.8 and 2.9 Ma, its fate is largely unknown because of a regional hiatus in the sedimentary record of the Hadar Basin between 2.9 and 2.7 Ma.
Our geological work offers new evidence for the northeast migration of the Hadar Basin, expanding our knowledge of the history of the basin substantially. Three new fossiliferous horizons with differing faunal composition have been identified suggesting in situ faunal change. While the fauna in the older unit is comparable to that at Hadar and Dikika, the younger units contain species that indicate more open conditions along with remains of Homo. New data from Mille-Logya reveal how hominins and other fauna responded to environmental changes during this key period. Our results show a connection between geotectonics, sedimentary basin migration and an in situ faunal change.
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To achieve this, it is crucial not only to establish chronological relationships between fossiliferous sites, but to investigate stratigraphic, structural and facies relationships between discontinuous exposures of sedimentary basins. Early geological maps of the region showed isolated Plio-Pleistocene sediments within the new site, amidst basalt flows attributed to the Afar Stratoid Series.
In these broad-scale maps, the sediments were attributed to undifferentiated Quaternary strata or the White Series (Enkafala Beds; both mapping units, were also used to indicate outcrops of the Hadar Formation, the latter having been much more thoroughly scrutinized since initial fossil discoveries at the Hadar site. Sediments in areas nearby broadly bracket and partly overlap with the strata of the Hadar Formation.
Sedimentary exposures in the Mille-Logya area provide access to generally disconnected sections of up to ~60 m in total thickness. Between these discontinuous exposures, extensive colluvial cover of volcanic, boulder- to cobble-sized material obscures most outcrop. Furthermore, a number of post-depositional faults divide the exposures into disconnected fault-bounded blocks. Hence, our stratigraphic interpretations of relationships between sections are presently based on widespread marker beds, chemical groupings of interfingered basalts and tephras, nine new 40Ar/39Ar dates, and several magnetostratigraphic sections.
These observations are sufficient to describe the overall stratigraphy, and to divide the sedimentary strata into three main fossiliferous intervals each exposed at one of the three main areas: Gafura, Seraitu, and Uraitele.
Stratigraphic Analysis of Mille-Logya
The lowest stratigraphic unit, Gafura, begins with a sequence of thick, columnar-jointed basalt flows with intra-flow residual paleosols developed on the basalts. The Gafura sediments are poorly exposed, but occur along the southwestern flank of Iki-Ilu Ridge, and are best represented by a section exposed at Sidiha Koma (section JGW15-1). The upper surface of the basalt flow at the base of this zone forms a broad low-lying surface, dissected by the Gafura River, extending into the base of the Daamé Valley.
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This sequence of basalts defines the GFB-I and GFB-II groups (GFB = Gafura Basalts); it underlies the main sedimentary sequence and is thus stratigraphically distinct from the overlying flows represented as the Afar Stratoid Series. Within the lowermost exposures of the Gafura Basalts, a normal to reverse magnetostratigraphic reversal is recorded (see Supplementary Table 5). The transition to overlying sediments of the Sidiha Koma area is marked by mudstones with ferruginized burrows interspersed with thin, poorly-sorted sands with a framework of basaltic lithic grains, occasionally containing abundant gastropods, and some bivalves.
Near the top of Gafura sediments, additional basalt flows overlie the sediments locally, although these have not yet been attributed to one of the geochemically-defined groups (see Supplementary Figs. 1, 2 and Supplementary Table 1). The fossiliferous sediments of Gafura underlie a widespread diatomaceous unit, the Iki-Ilu Diatomite, which can be mapped in regionally extensive exposures along the southwestern flank of Iki-Ilu Ridge, across its southern tip, and into the floor of the Seraitu Valley, making this a practical stratigraphic boundary.
The middle stratigraphic unit is represented by the Seraitu lake beds, which often form bare, steep cliffs of largely mudstone outcrops representing lacustrine deposition. We use the base of the Iki-Ilu Diatomite as the lower boundary of this zone, although most sections cannot be mapped in continuity within a measured stratigraphic distance to the diatomite. The upper boundary of this zone may not be defined by a single widespread marker but can be locally taken as the stratigraphically lowest basalt flow with chemical composition characteristic of the UGB Group (Uraitele-Garsele Dora Group), which may consist of several different flow units.
The UGB is frequently accompanied by an overlying, distinctive and widespread air-fall tuff with well-preserved glass and small lapilli-sized pumices, the Goyana Tuff (see Supplementary Table 2). The sediments of the Seraitu lake beds are predominantly laminated clays which often contain abundant ostracods, gastropods and some bivalves, as well as plant fragments and fish remains. Diatoms in the Iki-Ilu Diatomite are somewhat recrystallized but identifiable to the genus Aulacoseira.
Tephras are also numerous in the lake beds but characteristically thin (<30 cm), often air-fall occurrences, in which the primary glass is altered. Despite this, abundant feldspar crystal populations are preserved, providing two of the 40Ar/39Ar dates reported here. Besides the Hinti Mageta Tuff at the base of the Seraitu lake beds, two tuffs within the lake sediments provide precise ages: MLP14/SR-6 at 2.576 ± 0.008 Ma and MLP14/GOY-2 at 2.485 ± 0.018 Ma.
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The third unit, Uraitele, includes limited sedimentary exposures in-between extensive and thick basalt flows of the UGB, GYB-I, and GYB-II groups, which outcrop in the Goyana and Uraitele areas. The sediments include some lenticular sandstones interpreted as fluvial channels, but are predominantly laminated mudstones with occasional gastropod and bivalve bearing sandstones formed on surfaces of the UGB basalt or within the mudstones.
The upper boundary of the Uraitele zone is as yet undefined, as the section continues in a thick sequence of numerous basalt flows that extend into the ridges of the Magenta Mountains at the northern extent of the area (mapped as the Afar Stratoid Series). As with the Gafura area, the sediments of the Uraitele area contain a number of reworked vitric tuffs generally lacking large feldspar populations, but having distinctive chemical composition with no known correlates from the Awash Valley (see Supplementary Table 2).
Given the above stratigraphic sequence, we can make some important interpretations of the basin’s history. Prior to ~3 Ma, there is no evidence of an active depositional basin in the Mille-Logya region. Thus, during the period characteristic of most active lacustrine sedimentation at Hadar and Dikika (~3.6-3 Ma), the sequence of Gafura Basalts and residual paleosols at Mille-Logya suggests subaerial emplacement of basaltic lavas followed by periods of non-deposition and pedogenesis.
The local onset of active subsidence and sedimentation is marked by early onset of shoreline and shallow lacustrine deposits overlying the uppermost GFB-II Group. The subsequent lacustrine sequence culminates in a deep, well-mixed lake represented by an Aulacoseira-dominated diatom facies within the Iki-Ilu Diatomite and invertebrate fossil rich mudstones preserved throughout the Seraitu lake beds zone. Few terrestrial indicators are present except occasional coarser-grained facies suggestive of shorelines or brief subaerial exposure, where occasional rhizoliths are preserved and vertebrate fossils are slightly more common.
This lacustrine setting is persistent throughout the exposures and continues into the overlying Uraitele sediments, the lowermost of which are characterized by gastropod-bearing shoreline facies developed on the surface of basalt flows.
Archaeological Survey
Archaeological survey was conducted in conjunction with paleontological reconnaissance. Archaeological visibility is extremely low due to the combination of a thick colluvial cover, relatively few exposed sections and the fact that most of the sedimentary deposits are lacustrine in origin. Nevertheless, all of the fossiliferous localities and their surroundings were examined on multiple occasions.
In general, lithic artifacts are infrequent and scattered at very low density. The only exception comes from Seraitu Dida where slopes on two adjacent ridges with sediments above the Uraitele Tuff have numerous handaxes and Levallois cores and flakes made on a fairly consistent coarse-grained but good quality volcanic material. While artifact densities were relatively high in this area, no concentrations suggested a source for this material that has temporal constraint.
Nevertheless, we excavated three trenches at the crest of one of the ridges above the Uraitele Tuff. In one of these trenches, about one meter into a layer of gravel, we found a single Levallois flake. A maximum age is provided by the Uraitele Tuff (2.443 Ma) but the minimum age remains unconstrained.
Fossil Discoveries
The aforementioned stratigraphic setting provides a framework to interpret fossil data recovered from the three units. Fossil concentrations at Mille-Logya are sparse and relatively difficult to locate. Yet, after four field seasons, the fossil collection currently includes 2287 specimens, of which 1835 were collected while the rest were observed and documented on site (Table 1). Fossil collections at MLP followed a standardized protocol in order to minimize collection bias.
The identifiable specimens comprise 62 Cercopithecidae, 4 Hominidae, 33 Proboscidea, 10 Camelidae, 165 Suidae, 135 Hippopotamidae, 36 Giraffidae, 944 Bovidae, 218 Equidae, 21 Rhinocerotidae, 20 Carnivora, 17 birds, and some rodents, fishes, turtles, and crocodiles.
Hominins were recovered from four different localities and are represented by a left and right proximal ulnae (MLP-786 & MLP-1617 respectively (Fig. 3a, b): 2.6-2.8 Ma; from two different localities, thus not from the same individual), a calvarium fragment (MLP-1469 (Fig. 3c): 2.6-2.8 Ma) and a diagnostic and complete upper second molar...
This research not only sheds light on human evolution but also highlights the geological connections between regions like the Awash River basin in Ethiopia and areas like Lancaster, PA. By studying the migration of sedimentary basins and the associated faunal changes, scientists can better understand the complex interplay between environmental factors and the development of early hominins.
Fig. 5. Results demonstrate that the Gafura assemblage is distinct from the Seraitu and Uraitele assemblages, with Gafura showing a high abundance of Notochoerus, and the Seraitu and Uraitele assemblages showing a high abundance of Alcelaphini and Antilopini, which are open-habitat indicator taxa.
The study of river networks extends beyond specific geographical locations, offering insights into broader ecological principles. Research conducted by scientists at institutions like Lancaster University has contributed significantly to understanding the dynamics of riverine ecosystems.
The river continuum concept, introduced by Vannote et al., provides a framework for understanding how biological communities change along the length of a river. This concept has been further developed and refined by researchers studying river networks as ecological corridors. Rodriguez-Iturbe et al. emphasize the importance of integrating hydrologic, geomorphologic, and ecologic dynamics to understand river networks as complex systems.
These studies demonstrate that the Gafura assemblage is distinct from the Seraitu and Uraitele assemblages, with Gafura showing a high abundance of Notochoerus, and the Seraitu and Uraitele assemblages showing a high abundance of Alcelaphini and Antilopini, which are open-habitat indicator taxa.
The legacy of milldams, as studied by Walter and Merritts, highlights the long-term impacts of human activities on river systems. Breaching milldams can lead to channel incision and stream bank erosion, affecting nutrient loads and ecosystem health. Understanding these processes is crucial for effective stream restoration and water quality management.
Table 1. Fossil Collections at Mille-Logya
| Taxon | Number of Specimens |
|---|---|
| Cercopithecidae | 62 |
| Hominidae | 4 |
| Proboscidea | 33 |
| Camelidae | 10 |
| Suidae | 165 |
| Hippopotamidae | 135 |
| Giraffidae | 36 |
| Bovidae | 944 |
| Equidae | 218 |
| Rhinocerotidae | 21 |
| Carnivora | 20 |
| Birds | 17 |
| Other | Various |
