https://www.science.org/doi/10.1126/science.abq2835
The evolution of hominoid locomotor versatility:
Evidence from Moroto, a 21 Ma site in Uganda
MacLatchy et al., Science 380, 172 (2023 14 April 2023
INTRODUCTION:
Inherent in traditional views of ape origins is the
idea that, like living apes, early large-bodied apes
lived in tropical forests. In response to constraints
related to locomoting in forest canopies, it has been
proposed that early apes evolved their quintessential
upright torsos and acrobatic climbing and suspensory
abilities, enhancing their locomotor versatility, to
distribute their weight among small supports and thus
reach ripe fruit in the terminal branches. This
feeding and locomotor transition from a quadruped
with a horizontal torso is thought to have occurred
in the Middle Miocene due to an increasingly seasonal
climate and feeding competition from evolving monkeys.
...
RESULTS: A short, strong femur biomechanically
favorable to vertical climbing and a vertebra
indicating a dorsostable lower back
confirmthat ape fossils fromMoroto II shared
locomotor traits with living apes. Both
Morotopithecus and a smaller ape from the
site have elongated molars with well-developed
crests for shearing leaves. Carbon isotopic
signatures of the enamel of these apes and of
other fossil mammals indicate that some mammals
consistently fed onwater-stressedC3 plants,
and possibly also C4 vegetation, in a woodland
setting. Carbon isotope values of pedogenic
carbonates, paleosol organic matter, and plant
waxes all point to substantial C4 grass biomass
on the landscape. Analysis of paleosols also
indicates subhumid, strongly seasonal rainfall,
and phytolith assemblages include forms from
both aridadapted C4 grasses and forest-indicator
plants.
CONCLUSION: The ancient co-occurrence of
dental specializations for leaf eating, rather
than ripe fruit consumption, along with apelike
locomotor abilities counters the predictions
of the terminal branch forest frugivory
hypotheses. The combined paleoecological
evidence situates Morotopithecus in a woodland
with a broken canopy and substantial
grass understory including C4 species. These
findings call for a new paradigm for the
evolutionary origins of early apes. We propose
that seasonal, wooded environments may have
exerted previously unrecognized selective
pressures in the evolution of arboreal apes.
For example, some apes may have needed to
access leaves in the higher canopy in times of
low fruit availability and to be adept at
ascending and descending from trees that lacked
a continuous canopy.
Abstract
Living hominoids are distinguished by upright
torsos and versatile locomotion. It is
hypothesized that these features evolved for
feeding on fruit from terminal branches in
forests. To investigate the evolutionary context
of hominoid adaptive origins, we analyzed
multiple paleoenvironmental proxies in
conjunction with hominoid fossils from the
Moroto II site in Uganda. The data indicate
seasonally dry woodlands with the earliest
evidence of abundant C4 grasses in Africa based
on a confirmed age of 21 million years ago (Ma).
We demonstrate that the leaf-eating hominoid
Morotopithecus consumed water-stressed
vegetation, and postcrania from the site
indicate ape-like locomotor adaptations. These
findings suggest that the origin of hominoid
locomotor versatility is associated with
foraging on leaves in heterogeneous, open
woodlands rather than forests.
"Compositionally, this assemblage has affinities
with modern phytolith assemblages fromwooded
grasslands, but it is also consistent with open
forests containing a substantial grass understory
(e.g., riparian forest in a savanna landscape,
dry forest, or forest openings)."
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