This is a global proxy temperature curve for the last 65 million years.
It has been constructed on the basis of delta oxygen18-values from the shells of deep sea
bottom-dwelling foraminifera from all over the planet.
The delta oxygen18-values of the shells of these single-celled organisms record the ambient
temperature of their environment, as you may recall from the previous lecture on climate
change. The origin, evolution and diversity of primates
throughout the last 55 million years were influenced by the effects of climate.
We will not go into too many details of this curve, but you will notice that on the left-hand
side of the curve that global temperatures were much higher 65 million years ago.
This was at the end of the Cretaceous period and the beginning of the Tertiary.
Continuing towards the right on the curve and towards recent times, you can observe
that there has been an overall trend towards a steady drop in global temperature.
The first event on the "curve" which we will visit, took place some 55 million years ago
at the boundary between the geological epochs of the Paleocene and Eocene.
Here, the records indicate that deep-sea temperatures may have increased between 3 and 4 degrees
centigrade in as little as 10,000 years. This event is called the Paleocene-Eocene
Thermal Maximum. The reason for the increase is most likely
massive volcanic eruptions in the nascent North Atlantic between Greenland and Norway.
The eruptions emitted large quantity of CO2 into the atmosphere, heating it, and may further
have triggered the release of methane - another effective greenhouse gas - from deepwater
methane hydrates ice on the seafloor. The cumulative effect of this on the Earth's
environment was rampant global warming. Coincident with this global warming event,
we find the first appearance of primates in the fossil record - the first time we find
their fossilized remains. Other mammal groups, such as even-toed and
odd-toed ungulates, also appeared at exactly this moment in time, almost simultaneously
in the northern continents of North America, Europe and Asia.
The mechanism under which global warming may have led to the origin of new lineages among
mammals, is not understood at the moment. A few million years after the Paleocene-Eocene
Thermal Maximum, temperatures remained high and subtropical and tropical evergreen forests
were widespread. Fossil finds indicate that this was crucial
to the primates, as these environments allowed them to diversify rapidly into several lineages,
both living and extinct. It is in the beginning of the Eocene epoch
that we find the roots of the two major divisions within the primates; the Strepsirrhini and
the Haplorrhini. Respectively, the "wet noses" and the "dry
noses" that you may recall from the previous lecture.
The suborder Strepsirrhini consists among others of the living lemurs and the lorises,
as well as the now extinct adapiforms. The other major radiation of primates in the
early Eocene was the Haplorrhini or "dry noses". This very successful group includes recent
Tarsiers, New World monkeys, and our own group, the Old World monkeys.
Fossils referable to several haplorrhine lineages appear at the very beginning of the Eocene
epoch, and the entire suborder was very diverse throughout most of the Eocene.
However, following the Early Eocene Climatic Optimum, global temperatures fell gradually,
and the climate became drier and cooler. This negatively affected the subtropical and
tropical forests, and primate diversity went into a worldwide decline.
This observation is supported by a decrease in the relative number of fossils towards
the end of the Eocene. The plesiadapiforms - their sister-group suffered
worse and became totally extinct. Some 33 million years ago, at the Eocene-Oligocene
boundary, global temperatures plunged due to the formation of the first Antarctic ice
caps. This event radically changed global marine
circulation patterns, as well as resulting in a worldwide drop in sea level.
This led to a minor mass extinction event. It also thrust the planet into the cool and
dry "icehouse" climate, which has lasted until recent times.
The following Oligocene epoch - lasting between approximately 33 and 23 million years ago
- is marked by an all-time low in the fossil record of primates.
Their natural habitats of warm, subtropical broadleaved forest were severely restrained
in the cool and dry Oligocene epoch. However, it is also during the Oligocene epoch
that we find the first evidence for the origin of the catarrhine primates - also called the
Old World monkeys. Their colloquial name stems from the reason
that all recent members live in Africa and Asia - "the Old World".
And this is the group to which we humans belong. The fossil evidence for this is the skull
of Saadanius, a fossil primate from Saudi Arabia which lived around 28 million years
ago. Saadanius displays a number of features, which
indicate it as intermediate between catharrhines and their nearest extinct relatives.
We are now in the exhibitions of the Natural History Museum of Denmark.
On the podium next to me are skeletons of various extinct and living members of the
Hominidae - the great apes. The earliest fossils of this group derive
from middle of the Miocene epoch some 12 million years ago.
Living members of the group include the orang-outans, gorillas, chimpanzees and humans.