The question of whether the essential characteristics that characterize
placental animals
like us evolved in our ancestors before or after the extinction catastrophe
that exterminated the dinosaurs has been a source of much controversy.
Following an investigation by academics from the University of Fribourg in
Switzerland and the University of Bristol in the UK, that argument may now
have finally been resolved.
Prior to the 66 million-year-old, dinosaur-killing Cretaceous-Paleogene
(K-Pg) mass extinction, no conclusive
placental mammal remains
have been discovered. However,
molecular clock evidence
from the fossil record reveals that the lineage dates back even earlier, to
the period of the dinosaurs.
In order to identify the common ancestors of species, analyses of molecular
clock data 'wind back' genetic changes that happen continuously through
time.
Researchers have demonstrated how the earliest placental mammals most
likely first appeared during the Cretaceous period, briefly coexisting with
dinosaurs thanks to the use of a novel statistical analysis technique.
According to Emily Carlisle, a paleobiologist at the University of Bristol,
"we gathered thousands of fossils of placental mammals and were able to see
the patterns of origination and extinction of the different groups."
We could estimate the time of placental animals' evolution based on
this.
The researchers' model also demonstrates that more contemporary lineages of
placental animals didn't begin to appear until after the asteroid impact.
Therefore, it's probable that after the dinosaurs (and a large number of
other species) were extinct, the conditions were better for
diversification.
Based on a Bayesian Brownian bridge model, the ages of clades—groups of
creatures having a common ancestor—were calculated. This kind of statistical
model uses probability to identify evolutionary trends over time spans in
the absence of direct evidence.
The researchers calculate that 21.3 percent of the 380 placental mammal
groups in their dataset may have a Cretaceous origin.
This comprised the ancestors of dogs, cats, rabbits, and hares, as well as
the primates. Furthermore, the simulations agreed well
with other molecular clock findings that point to equally early origins for placental animals.
"The model we used estimates origination ages based on when lineages first
appear in the fossil record and the pattern of species diversity through
time for the lineage,"
explains
evolutionary scientist Daniele Silvestro from the University of Fribourg in
Switzerland. Based on the group's last sightings, it may also calculate the
age of extinction.
The team
contends
that, in cases when there are few fossils available, the model utilized here
is more reliable than utilizing genomic data or fossil records to determine
a species' evolutionary routes.
It's probably not unexpected that these placental mammals don't show up in
the record in their earliest forms as relatively few creatures ever attain
fossil status since preserving an organism as a fossil requires a very
specific set of circumstances.
The researchers are now hopeful that additional studies will be able to use
the model they have established. The outcomes obtained by this statistical
technique should get better as more work is done on organism categorization
and digitizing fossils.
According
to University of Bristol paleobiologist Phil Donoghue, "By examining both
origins and extinctions, we can more clearly see the impact of events like
the K-Pg mass extinction or the Paleocene-Eocene Thermal Maximum
(PETM)."
The research has been published in
Current Biology.
