Relevance of the eastern African coastal forest for early hominin biogeography
Josephine CA Joordens cs 2019 JHE 131:176-202
doi org/10.1016/j.jhevol.2019.03.012 open access
The influence of climate change on hominin evolution is much debated.
2 issues hamper our understanding of this process:
- the limited hominin fossil record,
- incomplete knowledge about hominin spatial occupation of Africa.
Here, we analyze the presently known hominin fossil distribution pattern,
we explore the potential geographic distribution of hominins between ∼4.5 & ∼2.5 Ma.
We focus on assessing the relevance of the Coastal Forest of E-Africa (CFEA) along the Indian Ocean as a core area for early hominin evolution.
Based on bio-geographic-phylo-geographic data, we propose the coastal refuge hypothesis:
the CFEA provided a refugium for early hominins in periods of variable climate & strong seasonality during eccentricity maxima.
From this refuge, evolved spp could disperse inland (e.g. to rift basins) via vegetated humid corridors, whenever onset of stable climate periods with low seasonality during eccentricity minima allowed expansion out of the coastal enclave.
We develop a conceptual model in time & space, comparing predictions with climatic & hominin fossil records.
The results imply
1) between ∼4.5 & 3 Ma, ongoing (mostly anagenetic) hominin evolution occurred in the CFEA, punctuated by inland dispersal events at ∼4.4, 4.2, 3.8, 3.5 & 3.2 Ma;
2) before ∼3 Ma, the Afar Basin was a (sub)core area often connected to & rel.similar to the CFEA, while other inland areas were more or less marginal for early hominin habitation;
3) after ∼3 Ma, N-Hemisphere Glaciation exerted strong influence by causing latitudinal contraction of the CFEA, leading to habitat fragmentation, isolation of hominin populations & possible cladogenetic evolution.
A major challenge for the coastal refuge model is:
at present, no (hominin) fossils are known from the CFEA.
We consider how this can be explained, and possibly overcome with targeted search efforts.
Furthermore we discuss how the model can be
- tested, e.g. with molecular phylo-geography approaches,
- used to predict new hominin fossil locations.
With this study, we hope to contribute a fresh perspective to the climate-evolution debate, emphasizing the role of climatic stability, length of dry season & vegetation cover, to facilitate connectivity between hominin core & marginal habitats.
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