Carnegie scientist Chris Beard discovers a new rung on the ladder of evolution
Carnegie paleontologist Chris Beard tells how he made the scientific and political
choices governing his selection of fossil research sites, and what it’s like to discover
a very early rung on the ladder of human evolution.
By Chris Beard
Toward the end of The Origin of Species, his landmark treatise that made
the theory of evolution one of the most influential ideas in modern science, Charles
Darwin wanted to give his concept a humanistic slant. Darwin knew that his new theory,
which he had so elegantly supported using examples from Galapagos Islands finches,
domestic animals and many more obscure representatives of the animal kingdom, applied
equally to humans. His problem was to make this point without overly offending the
entrenched cultural and religious beliefs of Victorian England.
Darwin decided to approach the subject stealthily-he simply noted that African
apes, which were still only poorly known to Western science, suggested that the original
cradle of humanity might well have been the African continent. Thus in one stroke,
Darwin accurately anticipated later fossil discoveries, such as the Taung baby, Lucy,
and other early hominids, which most scientists now agree pinpoint Africa as the ancestral
homeland of humankind.
Thanks to the work of noted paleoanthropologists such as Raymond Dart, Louis and
Richard Leakey, and my former professor Alan Walker, we now know that humans evolved
from ape-like ancestors that lived in Africa 5-10 million years ago. Transitional
forms that document this evolutionary progression, known by such formidable scientific
names as Ardipithecus ramidus, Australopithecus afarensis and Homo erectus, show that
early human ancestors evolved the capacity to walk on two legs (bipedalism) millions
of years before their brains grew larger than those of chimpanzees. Although many
gaps remain, in the grand scheme of things we know how natural selection transformed
our ape-like ancestors into the thoughtful beings we are today.
Unfortunately, much less is known about more distant phases in our evolutionary
history. Humans belong to the order of mammals known as Primates, which are distinguished
from other mammals by having eye sockets that are surrounded by bony struts or plates,
nails rather than claws or hooves, and grasping big toes. Humans are the only primates
that lack grasping big toes, but fossils show that our ape-like ancestors had them.
From a zoological standpoint, two major groups of primates are alive today. One
is the “lower” or prosimian primates-the scientific term prosimian derives
from the Latin pro- (before) and simian (ape), meaning “before the apes.”
These lower primates include the lemurs of Madagascar, the bushbabies and pottos of
mainland Africa, the lorises of southern Asia, and the tarsiers of southeast Asian
islands. More familiar to us today are the “higher” or anthropoid primates-
anthropoid derives from the Greek anthropos for human being. This group of primates
includes humans, apes and the monkeys of both the New and Old Worlds.
A wide anatomical and ecological gulf separates lower and higher primates today.
Lower primates tend to be nocturnal, move by leaping from one vertical tree trunk
to the next, and often live in simple social groups. In contrast, higher primates
are primarily active during daylight hours, move quadrupedally along the tops of branches
or else by knuckle-walking on the ground, and typically live in complex social settings
comprising many individuals.
In order to understand the full story of how humans came to exist, a major goal
of primate paleontology is to bridge the wide evolutionary gap between living lower
and higher primates. Otherwise, a critical transition in our evolutionary history
would remain unknown. Imagine trying to document the history of Western Civilization
if we knew nothing about Plato, Aristotle or the rise and fall of the Roman Empire.
Since coming to the Carnegie Museum of Natural History in 1989, a major focus of
my research has been to shed new light on the origins of higher primates. To achieve
this, I often spend three months or more each year in the field, searching for critical
fossils to help fill the gap separating lower and higher primates. All of this time
in the field has paid off handsomely, because the results of these Carnegie Museum
expeditions have fundamentally changed the way scientists think about our distant
higher primate ancestors.
An Earlier Date for Higher Primate Origins
Our first breakthrough was the discovery of six skulls of the 50-million-year-old
primate Shoshonius cooperi in the Wind River Basin of central Wyoming. Shoshonius
belongs to an extinct group of primates called the Omomyidae, a group that many experts
believe includes the ancestors of both modern tarsiers and higher primates. Our discoveries
of Shoshonius in the Wind River Basin marked the first time in more than 100 years
that skulls of such ancient omomyid primates had been found in North America. Indeed,
only once before had a similar omomyid skull been found on this continent. In 1880
Jacob Wortman, who then worked for the American Museum of Natural History (Wortman
was subsequently employed by the Carnegie Museum for a short time), rode on horseback
into the badlands of Wyoming’s Bighorn Basin and found a single skull of the omomyid
primate Tetonius homunculus. While Wortman’s Tetonius skull has intrigued paleontologists
for more than a century, it also left a great deal to be desired because the specimen
was partly crushed during fossilization and critical parts of the skull are missing.
In contrast, some of our more recently discovered Shoshonius skulls were beautifully
preserved, revealing numerous aspects of anatomy in intricate detail.
Our precious new skulls showed that Shoshonius was a far more specialized primate
than its 50-million-year age might imply. Like modern tarsiers, Shoshonius possessed
remarkably large eye sockets. Even more convincing was the fact that Shoshonius and
tarsiers share numerous details of the ear region that are never encountered in other
living or fossil primates. Because the ear region of mammals is such an anatomically
complex part of the skull, with numerous tiny bones, nerves and blood vessels crowded
into a small space, paleontologists have long relied on this area as a guide to evolutionary
relationships.
Based on the number and intricacy of anatomical details shared by Shoshonius and
tarsiers, the conclusion was inescapable. Shoshonius was too specialized along the
evolutionary path leading to tarsiers to have been ancestral to both tarsiers and
higher primates. More importantly, by demonstrating that the lineage that ultimately
gave rise to modern tarsiers was already established 50 million years ago (the age
of Shoshonius), it became possible to place new limits on the timing of higher primate
origins.
Of all the living lower primates, tarsiers alone stand as the nearest living relatives
of higher primates. Unlike lemurs, lorises and other lower primates, tarsiers and
higher primates share a wide variety of anatomical attributes. For example, the noses
of lemurs resemble those of cats, dogs and other mammals in being wet and hairless
and having a minute vertical slit running from between the nostrils to the underside
of the upper lip. In contrast, tarsier noses resemble those of higher primates (including
humans). The enlarged eyes of tarsiers resemble those of higher primates in having
an area of increased visual acuity known as the retinal fovea and in lacking a tapetum
lucidum. The tapetum lucidum, which occurs in lemurs and other lower primates, as
well as cats, dogs and many other mammals, is responsible for the familiar “glow
in the dark” phenomenon that results when lights shine on their eyes at night.
Even more convincing, studies of tarsier and higher primate DNA reveal that tarsiers
and higher primates share a more recent common ancestor than either does with lemurs
and other lower primates. Given the evidence that tarsiers are the nearest living
relatives of higher primates, calibration of the origin of the tarsier lineage at
50 million years ago based on Shoshonius meant that the higher primate lineage must
have been in existence by then as well.
When my colleagues and I published our original study of Shoshonius in 1991, the
only problem with our interpretation was that it conflicted with the fossil record
of higher primates themselves. The oldest higher primates known at that time were
a mere 35 million years old, some 15 million years younger than our minimum estimate
based on Shoshonius!
Paleontologists do think of time differently than most people-but 15 million years
is a long interval by any definition. Unless we were willing to concede that Shoshonius
was not as closely related to tarsiers as we thought (we weren’t), or that the evidence
suggesting that tarsiers are the nearest living relatives of higher primates was flawed
(it seemed strong), we had to conclude that the fossil record of higher primates themselves
was yielding spuriously young estimates of their antiquity. This seemed the lesser
evil at the time, so we suggested that early ancestors of higher primates did indeed
exist 50 million years ago, but that fossils of these early higher primates had so
far eluded paleontologists. Accordingly, finding actual fossils to document this missing
“phantom lineage” of higher primates became my top research priority. But
where in the world should I go to search for such elusive fossils as these?
Out
of Africa and Beyond
At the time, the oldest higher primate fossils were from North Africa (the Fayum
Depression of Egypt), so the safest strategy would be to search for older African
fossil sites with the potential to yield even earlier fossils. Indeed, one of my close
friends and colleagues, Dr. Marc Godinot of France’s Centre National de la Recherche
Scientifique in Montpellier, had just found a few tantalizing teeth that were perhaps
as old as 40-45 million years at a locality called Glib Zegdou in western Algeria.
Marc showed me these priceless specimens in 1991, and both of us agreed that they
were the fossil molars of a tiny higher primate much older than those from Egypt-a
species Marc later named Algeripithecus minutus because of its small size.
Could it be that the entire story of primate and human evolution was like watching
the movie “Out of Africa” over and over, with every major transition in
primate evolution occurring on that continent? While I was then, and remain, intrigued
by this possibility, political developments in the early 1990’s made a major research
project in the Sahara Desert unlikely.
Beyond the political difficulties in Africa, several factors made Asia a more promising
place to search for the phantom lineage of higher primates that Shoshonius suggested
must exist. First, in the same way that Charles Darwin had used the living African
apes to speculate that Africa was the geographic source of humans, one could point
to the southeast Asian tarsiers as evidence that the evolutionary split between the
tarsier and higher primate lineages may well have occurred in Asia. Moreover, several
controversial fossil primates were known from the interval of 40-45 million years
ago in Asia, and these fragmentary fossils had repeatedly been cited as potentially
belonging to early higher primates. All that was necessary to test this possibility
was better knowledge of these and other fossil primates from Asia.
My colleague Mary Dawson and I decided that China was the most favorable Asian
country in which to launch this search, because of its relatively rich, yet largely
untapped, fossil record, and because we had longstanding friends and colleagues there
who were anxious to join in our search for the phantom lineage. Accordingly, we have
mounted cooperative expeditions to China with the Institute of Vertebrate Paleontology
& Paleoanthropology in Beijing each year since 1992.
Although our research projects in China have taken us to such exotic locales as
the Turfan Basin in Xinjiang Uygur Autonomous Region (along the ancient “Silk
Road”) and the Baise Basin in Guangxi Zhuang Autonomous Region near the Vietnamese
frontier, our two most productive study areas occur in China’s heartland. Just west
of Shanghai is a commercial limestone quarry, which supplies raw materials for the
increased cement production that China’s rapid economic growth requires.
The limestone itself is of Triassic age-from the very beginning of the Age of Dinosaurs
some 220 million years ago-and is therefore much too old to yield fossils of the earliest
higher primates. But the chemical composition of limestone makes this rock dissolve
easily in rainwater, leading to “karstification” or the creation of numerous
small fissures or limestone caverns that are often much younger than the original
limestone itself. As chance would have it, the Triassic limestone in the quarry near
Shanghai is crisscrossed by fissures dating to the middle Eocene (some 45 million
years ago), in the middle of the interval in which the phantom lineage of higher primates
must have existed .With our Chinese colleagues Qi Tao and Wang Banyue, we harvested
the fossil-rich matrix within these fissures, screen-washed the matrix to separate
the fossils from the surrounding mud, and studied the prehistoric mammals yielded
by this labor- intensive process. Although our continuing field work in this limestone
quarry is hardly glamorous, it gave me the first glimpse at the phantom lineage of
higher primates I had been searching for.
The Dawn Monkey from China
The first specimens to document the phantom lineage of higher primates would not
have impressed anyone without years of training in primate dental and jaw anatomy.
Indeed, the best fossil I could find was a tiny lower jaw preserving the crowns of
three teeth-the last premolar and the first two molars. This specimen became the holotype,
or “type specimen,” of Eosimias sinensis (“dawn monkey from China”
in Latin and Greek) when my colleagues and I published our first scientific account
of our discovery in 1994.
Despite its modest appearance, Eosimias sinensis yielded great insight into the
nature and adaptations of the phantom lineage. First of all, it became clear that
the earliest higher primates were tiny, mouse-sized animals with a body mass of around
100 grams (about 3.5 ounces). This alone contradicted most predictions concerning
the earliest higher primates. Because modern monkeys and apes are generally larger
than lower primates, many scientists assumed that this size differential could be
traced back to the very beginnings of the higher primate lineage. In contrast, Eosimias
sinensis was as small as the smallest living monkey-the pygmy marmoset (Cebuella pygmaea)
of South America. Further, the three teeth preserved in the type specimen of Eosimias
sinensis were much more primitive than those of any of the early higher primates known
from Africa, including Algeripithecus minutus.
The fact that Eosimias was so much more primitive than roughly contemporary African
higher primates was potentially one of its most important attributes from a scientific
perspective. First, the presence of the most primitive higher primates in China could
mean that higher primates actually originated on the Asian landmass, thus bursting
the bubble of the “Out of Africa” hypothesis. Second, Eosimias and other
transitional fossil forms like it help decipher the precise pathways by which evolution
occurred. But the primitive nature of Eosimias turned out to be a double-edged sword.
Other scientists were not convinced that Eosimias possessed enough higher primate
features to be truly diagnostic, and one expert on early mammals went so far as to
proclaim that Eosimias was not a primate at all, but a hedgehog (hedgehogs are Old
World insectivores, closely related to shrews and moles). Obviously, we had to find
better, more nearly complete fossils of Eosimias and other primates like it to silence
these critics.
A Carnegie Centennial Dawn Monkey
So it was that we initiated a second major field project in China, this time in
the valley of the Yellow River in southern Shanxi Province. The Chinese government
had decided to build a dam on the Yellow River downstream, which would flood several
historically important fossil localities. With our Chinese colleagues Tong Yongsheng,
Wang Jingwen and Huang Xueshi, our task was to try and extract as much as possible
in the way of fossils and supporting data from this region before it becomes submerged
in 1997. I knew that the Yellow River localities had the potential to yield important
fossils bearing on the origin of higher primates. Indeed, the first primate ever to
be reported from the Eocene of China was an animal named Hoanghonius stehlinii, originally
described from this region in 1930. Since its initial description, Hoanghonius had
been compared favorably with higher primates, and many experts believed that Hoanghonius
was a better candidate than Eosimias for a transitional form between prosimians and
higher primates.
Fortune smiled upon us during our 1994 and 1995 field seasons in the Yellow River
valley. First, our field crews located exquisite new fossils of Hoanghonius, thus
proving that Hoanghonius is not closely related to higher primates after all. Second,
we discovered fossil primates that had never before been found in that region, including
several new species related to Eosimias, and the most nearly complete fossil tarsier
ever discovered. Then in May 1995, on the eve of the centennial celebration of The
Carnegie, our field team discovered virtually complete lower jaws representing a new
species of Eosimias-a species we decided to name Eosimias centennicus in honor of
our museum’s first 100 years of exploration and research in natural history. Because
of its relative completeness, Eosimias centennicus would finally answer the critics
who questioned whether Eosimias represented an early transitional phase in higher
primate evolution.
This new species showed many features that were unmistakably monkey-like. For example,
its chin was deep and robust like that of a monkey, and its canine teeth projected
high above the other teeth as often occurs in monkeys. In fact, if Eosimias centennicus
was as monkey-like as much of its anatomy suggested, we could even determine the sex
of the 40-million-year-old type specimen! In most living species of monkeys, males
have greatly enlarged canine teeth that they use to intimidate rival males, analogous
to the antlers of deer. If this pattern also held in Eosimias, our type specimen must
have been a male by virtue of its relatively enormous canines.
Given that our field work in China has yielded so much new information on the early
evolution of higher primates, one might ask whether much remains to be learned. Science
is a successive approximation of the truth, and the answers we have gained have already
led to many more questions. For example, we still have much to learn about the anatomy
of the skull and the limb bones of Eosimias. These details are necessary to give us
a more complete picture of the paleobiology of the earliest higher primates (Did Eosimias
leap like a lower primate or did it walk quadrupedally like a monkey? Was the brain
of Eosimias already enlarged beyond those of typical lower primates?). Moreover, given
the diversity among Eosimias and species like it in Asia (our teams have also found
many other new primates, but we haven’t named them yet), it is already clear that
we must search for even older fossils if we are to find the earliest representatives
of the higher primate lineage. Every time a paleontologist finds a fossil that fills
an evolutionary gap, two more gaps of smaller magnitude are created that also need
to be filled.
Here’s hoping that the second century of exploration and research at the Carnegie
Museum of Natural History is as distinguished at solving the mysteries of the natural
world as its first 100 years have been. Eosimias bicentennicus anyone?
Chris Beard is associate curator in the Section of Vertebrate Paleontology,
Carnegie Museum of Natural History.
