MISSING LINKS AND DAWN MONKEYS

What is unique about this particular ravine, though, is not the loess.
In this part of Shanxi Province, loess is ubiquitous, draping over older
geological features like autumn leaves covering a well-kept lawn. But here,
as the ravine approaches the Yellow River, it cuts deep into the loess. For
the last fifty yards or so of its existence, the ravine finally succeeds in
breaking through the loess altogether to expose the much older underlying
strata. Even to the untrained eye, it is clear that these rocks are different,
both in terms of their composition and their segregation into different
layers or beds. They consist of alternating bands of blue-green
mudstone, pale yellow and white limestone, and thick gray sandstones,
the last of which show internal evidence of stratification in the form of
minute swales of sand grains known as cross-bedding. The fossils we seek
are concentrated in the layers of mudstone and limestone. They are
roughly forty million years old, about six times older than the earliest
putative hominids ever discovered. They pertain to an interval of Earth
history known as the Eocene, the Greek roots of which translate more
or less as “dawn of recent [life].”
As its etymology suggests, the Eocene was a pivotal period in the history
of life on Earth—a time of transition from ancient to modern. The
earliest members of most living orders of mammals first appeared and
became geographically widespread, replacing more archaic forms that
left no living descendants. Such distinctive and highly specialized types
of modern mammals as bats and whales first showed up in the Eocene,
together with the earliest odd-toed ungulates (horses, rhinos, and tapirs),
even-toed ungulates (pigs, camels, and primitive relatives of deer and antelopes),
and others. The order of mammals to which we belong, the Primates,
also first became geographically widespread and ecologically
prominent at the beginning of the Eocene, although a few scattered fossils
hint that primates are somewhat older yet. At the same time, the
Eocene witnessed the decline and extinction of many groups of mam-

mals that first evolved alongside the dinosaurs, or immediately following
their demise. Examples include the vaguely rodentlike multituberculates,
the raccoon- or bearlike arctocyonids, and the large herbivores
known as pantodonts and uintatheres. The Eocene also witnessed a great
evolutionary diversification of flowering plants, together with the insects
that feed on them.3

MISSING LINKS AND DAWN MONKEYS

To the north and east, wheat fields extend across the plateau as far as
the eye can see. Immediately west of the ravine, the sleepy village of Zhaili
shelters the peasant farmers who tend the surrounding fields. A narrow
path, hewn into the western wall of the ravine, provides access to the
bottom some 150 feet below for the villagers and their sheep and goats.
Walking down this path, you can’t help but notice the peculiar nature of
the nearly vertical walls of the ravine. The rock defining both sides of
the ravine is soft and pliable, so easy to work that many people in this
part of China actually carve small caves into it, which function as storage
rooms or even small homes. Geologically, this type of rock is known
as loess. It is composed of wind-blown sediment laid down by countless
dust storms that swept across this part of China during the Pleistocene
Epoch, when vast ice sheets were expanding and contracting farther north
in Siberia.

MISSING LINKS AND DAWN MONKEYS

A fuller consideration of human origins requires us to place our own
evolutionary history within a broader context. Did humans take longer
to evolve our unique characteristics than other living primates, or did
our ancestors simply experience unusually high rates of evolution? For
that matter, how unique are humans with respect to other primates anyway?
Which seemingly “human” traits are ours alone, and which are
shared with various primate relatives? Where do humans lie on the family
tree of all primates, and what does that tree look like? Where do primates
lie on the larger family tree of all mammals? Were there particularly
critical events during the earlier phases of our evolutionary history,
before our own lineage branched away from those leading to chimpanzees
and other living primates? Today, these questions pose far greater scientific
challenges than simply filling in the constantly shrinking gaps in
the human fossil record. Yet, ironically, when most people hear the term
“missing link,” they think of a gap in the fossil record that supposedly
fails to link modern humans with our apelike ancestors. The dirty little
secret of paleoanthropology is that, while there are plenty of missing links,
they don’t occur where most people think they do. They exist farther
back in deep time. Ultimately, this is why I’m at the bottom of a ravine
on the banks of the Yellow River.
The ravine itself is a natural erosional feature, an ephemeral drainage
flowing into the Yellow River from the north. It dissects a relatively flat
plateau, which—like most rural parts of central China—is now under intensive
wheat cultivation. Standing on top of the plateau at the head of
the ravine offers a panoramic view of the surrounding terrain. To the
south, on the far side of the Yellow River in Henan Province, lie rugged
mountains composed primarily of limestone of Ordovician age. Some 450
million years ago—about twice the age of the earliest known dinosaurs—
the rock now forming the crest of this range was deposited in a warm,
shallow sea not unlike that surrounding the modern Bahamas.

MISSING LINKS AND DAWN MONKEYS

Paleoanthropology is the scientific study of human origins. In the
strictest sense, paleoanthropologists seek to illuminate the evolutionary
history of the human lineage as it evolved from our more apelike ancestors.
Fossil hominids are the crown jewels of paleoanthropology. Without
them, theories about when, where, and how our species evolved
would be helter-skelter, unconstrained by hard data. One of the great triumphs
of twentieth century science has been the recovery of an amazing
diversity of hominid fossils, mainly from eastern and southern Africa,
but also from various parts of Eurasia, ranging from France and Spain
to China and Indonesia. Discoveries of new fossil hominids continue unabated.
Considered as a whole, the fossil record of early humans is now

complete enough that, at least in broad strokes, we know how humans
evolved from more apelike precursors. Virtually all paleoanthropologists
agree, for example, that the human lineage originated sometime between
five and seven million years ago in Africa, and that early humans acquired
the ability to walk upright on two legs millions of years before their brains
enlarged much beyond those of chimpanzees.2

MISSING LINKS AND DAWN MONKEYS

My persistence is rewarded when I split apart another block of greenish-gray limestone. Inside I find a nearly complete maxilla, or upper jaw, of a small rodent, replete with three black teeth that glisten like fresh obsidian in the sunlight. Peering through a hand lens that I keep tied to a leather thong draped like a necklace under my tee shirt, the diagnostic pattern of cusps and crests on the fossilized teeth readily identifies the creature as Pappocricetodon schaubi. A primitive progenitor of modern mice, rats, and gerbils, Pappocricetodon is the most abundant fossil mammal known from this site.1 Though it’s not exactly the pivotal discovery I had hoped for, finding the mortal remains of any animal that lived millions of years ago invigorates the mind. I begin to contemplate the weighty scientific issues that have led me to travel halfway around the world, to this remote part of central China’s Shanxi Province.

My particular area of scientific expertise, vertebrate paleontology, is
in the midst of a sea change. Much of what I learned as a graduate student
is being challenged by provocative new fossils and new methods of
interpreting them, if not discarded altogether. Increasing globalization
and the collapse of the Soviet Union and its satellite states have opened
up most of the world to paleontological exploration, including places
that, only a few years earlier, I never dreamed of being able to visit in
search of fossils. On a separate front, molecular biologists are sequencing
the DNA of various organisms at an increasingly frenetic pace, churning
out megabytes of raw data that are being used to test old ideas, and
to propose new ones, about the evolutionary relationships of living plants
and animals. All in all, it feels like a unique moment in history and a
great time to be a paleontologist, especially when you’re involved in one
of the most exciting debates to hit the field of paleoanthropology in many
years.

Missing Links and Dawn Monkeys

Missing Links and Dawn Monkeys

In rural China, the highest compliment you can get is not that you’re
attractive or smart. It’s that you work really hard. As I shift to stay in
the scant midday shade offered by a deep ravine on the northern bank
of the Yellow River, this proletarian attitude makes a lot of sense. When
I left the United States earlier this month, spring had barely begun. Checking
the calendar in my field notebook, I see that it’s only mid May—too
early in the season for a heat wave. Yet for the past few days, my team
has endured triple digit temperatures. Each of us sports a tan several
shades deeper than our normal hue. A few yards away, where he chips
at a piece of freshwater limestone that just might contain a fossil, my
colleague Wang Jingwen is beginning to live up to his nickname, which
translates roughly as “black donkey.” I’m told that the local villagers have
been praising our work ethic, because when it gets this hot, even the peasants
take a siesta under a shade tree.
We have no choice but to tolerate the heat of the noon sun, because
it provides the best lighting conditions for finding fossils. At this time of
day, there are no shadows to hide the small jaws and limb bones that
have been entombed in these rock strata for the past forty thousand millennia
or so. Having traversed twelve time zones to get here, I’m not about
to forgo the chance to find an important specimen merely because of the

oppressive heat.

adopt a monkey for free

Missing Links and Dawn Monkeys