https://www.discovermagazine.com/the-sciences/human-ancestors-may-have-bucked-an-evolutionary-trend

In evolution, competition is thought to be a
zero-sum game. One species adapts and survives.
Another doesn’t and dies off. A new study in
Nature Ecology & Evolution posits that human
ancestors might be an exception.

Conventional wisdom in evolutionary theory
has held that climate has driven the rise and
fall of various hominin species. In most
vertebrates, interspecies competition also
plays an important role. That role has been
discounted in human ancestors, according to
the study.

“We have been ignoring the way competition
between species has shaped our own
evolutionary tree,” said Laura van Holstein,
a University of Cambridge archeologist and
author of the paper, in a press release. “The
effect of climate on hominin species is only
part of the story."

Van Holstein created a database of 385 known
hominin species — from Australopithecus sediba
to Homo floresiensis and noted the dates when
they lived. The researchers used a statistical
analysis model to investigate how competition
played a role in human ancestors’ evolution.

Van Holstein found that in many early
hominins — as in other mammals — separation
into other species increases, then flatlines,
at which point extinction rates start to ramp
up.

But when she looked at the later “Homo” groups
of hominins, van Holstein noticed a finding
she called bizarre. Her analysis showed that
competition between Homo species appeared to
result in even more species.

"This is almost unparalleled in evolutionary
science," van Holstein said.
....
So why the divergence? Later Homo sapiens
became ecosystem engineers, according to the
paper. Learning how to make and use tools and
to build fires gave later species adaptive
benefits that could improve quicker than any
evolutionary change.
....

https://www.nature.com/articles/s41559-024-02390-z
Diversity-dependent speciation and extinction
in hominins

Abstract
The search for drivers of hominin speciation
and extinction has tended to focus on the
impact of climate change. Far less attention
has been paid to the role of interspecific
competition. However, research across
vertebrates more broadly has shown that both
processes are often correlated with species
diversity, suggesting an important role for
interspecific competition. Here we ask
whether hominin speciation and extinction
conform to the expected patterns of negative
and positive diversity dependence,
respectively. We estimate speciation and
extinction rates from fossil occurrence data
with preservation variability priors in a
validated Bayesian framework and test whether
these rates are correlated with species
diversity. We supplement these analyses with
calculations of speciation rate across a
phylogeny, again testing whether these are
correlated with diversity. Our results are
consistent with clade-wide diversity limits
that governed speciation in hominins overall
but that were not quite reached by the
Australopithecus and Paranthropus subclade
before its extinction. Extinction was not
correlated with species diversity within the
Australopithecus and Paranthropus subclade
or within hominins overall; this is
concordant with climate playing a greater
part in hominin extinction than speciation.
By contrast, Homo is characterized by
positively diversity-dependent speciation
and negatively diversity-dependent
extinction—both exceedingly rare patterns
across all forms of life. The genus Homo
expands the set of reported associations
between diversity and macroevolution in
vertebrates, underscoring that the
relationship between diversity and
macroevolution is complex. These results
indicate an important, previously
underappreciated and comparatively unusual
role of biotic interactions in Homo
macroevolution, and speciation in
particular. The unusual and unexpected
patterns of diversity dependence in Homo
speciation and extinction may be a
consequence of repeated Homo range
expansions driven by interspecific
competition and made possible by recurrent
innovations in ecological strategies.
Exploring how hominin macroevolution fits
into the general vertebrate
macroevolutionary landscape has the
potential to offer new perspectives on
longstanding questions in vertebrate
evolution and shed new light on
evolutionary processes within our own
lineage.