Ancient footprints in Crete challenge theory of human evolution – but what actually made them?
Researchers have discovered some 50 footprints at
Trachilos in Crete that are nearly six million years old. It looks like they
may be from a hominin – a member of the human species after separation from the
chimpanzee lineage. But, as the authors point out themselves, the findings are
highly controversial – suggesting human ancestors may have existed in Crete at
the same time as they evolved in Africa.
So what should we make of it all? If the footprints
are confirmed to be from a hominin – additional studies are needed before we
can know for sure – it is unquestionably exciting.
The oldest footprints confirmed as hominin are the
Laetoli series, which date to 3.65 million years. The Laetoli series, found in
Laetoli, Tanzania, are now known to have been made by the early human ancestor
Australopithecus. It was up to six feet tall and had a foot function pretty
much indistinguishable from our own.
Laetoli footprints, the earliest known made by humans
The candidates
So what kind of two-legged creatures have roamed
Europe or nearby countries? We have abundant fossil evidence of great apes in
Europe at the time of the Trachilos footprints, but no confirmed cases of
hominins. Apes go as far back as 13m years ago, such as Pierolapithecus from
Barcelona. Two million years later, the pongine or orangutan relative
Hispanopithecus lived in the same region. Excellent skeletons of both indicate
they were probably partially walking upright.
The ape Dryopithecus and the possible hominin
Graecopithecus from Greece were also around. The latter is about seven million
years old, but unfortunately no skeleton has been found except for skull and
teeth. Slightly older at about nine million years old are the very complete
postcranial skeletons of Oreopithecus from Italy, which was unquestionably
walking on two legs – and probably in trees as well as on ground. We don’t know
for sure, but it might also be a hominin.
In Kenya, there was Orrorin, also slightly older than
Trachilos. It lived in trees but walked on two legs, completely upright.
Orrorin was quite likely a hominin or a very close relative of the common
ancestor of chimpanzees and humans, although human-like in all ways.
Importantly though, we unfortunately lack evidence of the feet, so we cannot
compare it with the Trachilos footprints.
Slightly younger than the Trachilos prints,
Ardipithecus (from Ethiopia) is a generally accepted member of the human
lineage. Like Orrorin it could have been close to the common chimpanzee-human
ancestor, but looked more like a modern human: homo sapien. It is becoming
increasingly clear that the common ancestor of humans and chimpanzees had limbs
and a trunk (a postcranial skeleton) much more like our own than like those of
living chimpanzees.
Footprints discovered in Trachilos
Gorilla prints?
So what or who made the Trachilos prints? They are
certainly convincing as real footprints, from the few pictures provided in the
paper. The age estimate of 5.7 million years also seems correct. The prints do
have a narrow heel compared to our general idea of what human footprints look
like, as the authors note. But that could easily be matched by the shape of
human footprints walking in wet mud, such as in an estuary – which may have
been the case. They have a big toe placed quite close to the others, like our
own, but so do the feet of gorillas.
Gorillas now appear to be in some respects a good
model for what the gait (and ecology) of the earliest human ancestors might
have been like, moving on two legs on the ground as well as in the trees.
The fact is that human footprints and foot function
vary enormously between steps as a consequence of the complexity of our anatomy
and ability to make choices from a large range of functional strategies to
maintain stability. Human foot pressure, which is the way force is applied over
the sole of the foot to the ground, overlaps with that of orangutans and pygmy
chimpanzees, and probably even more with that of gorillas. So in some
circumstances a human foot could look like that of a gorilla.
If all 50 of the Trachilos prints were made freely
available to other scientists as high resolution laser scans, we would have a
decent sample to assess their variability and compare them to other fossil and
recent footprints and foot pressure records. And indeed, the researchers behind
the study told The Conversation they are aiming to release all their data at
some point.
This would give us a good chance of saying who made
them. As it stands, they could as well be those of gorillas – which separated
from us over 10m years ago – as those of a member of our own human lineage such
as Oreopithecus or Orrorin.
Robin Crompton is an honorary research
fellow at the Institute of Ageing and Chronic Disease, University of Liverpool;
Susannah Thorpe is a reader in zoology at the University of Birmingham. This
article was originally published on The Conversation (theconversation.com)
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