WHAT
TIME WAS IT?
A year ago, on March 10, 2006 an exciting new exhibit opened at the Field Museum in Chicago. This exhibit is called “Evolving Planet”. It is the successor of a previous exhibit, “Life over Time”, which for years had showed visitors to the Museum how life had evolved on earth. With the passage of the years, “Life over Time” had become tired and somewhat outmoded. The Museum staff took five years to rework its setting and contents. The result, now named “Evolving Planet”, is designed to show the public how our world has developed from its beginnings 4.5 billion years ago. Have you seen this production? If not, I hope that this paper will encourage you to do so. It occupies a hall on the second floor of the Museum and is readily accessible seven days a week.
Any discussion of the development of our world must include three parts: the changing structure of the earth’s crust, the evolving life forms on it, and the time within which all of this took place. These elements are interdependent. The evolution of life forms needed time in order to unfold. As we shall see, the changing life forms as they became fossilized in rocks facilitated an interpretation of the age of those rocks. And the rock structure of the world itself has been molded and remolded over time. Time, structure of the earth’s crust, and life: These are the elements of “Evolving Planet”. In our discussion today we want to focus on the time it has taken the planet to evolve and the subdivisions of that time within which particular events occurred.
Curiosity about time on earth did not start with the construction of “Evolving Planet”. A Hindu creation myth tells us: “before time began there was no heaven, no earth and no space between. A vast dark ocean washed upon the shores of nothingness. A giant cobra floated on the waters. Asleep within its coils lay the Lord Vishnu.” The myth goes on to say how Vishnu created Brahma and how Brahma created the world – all in a remote mythic time.
The Judeo-Christian myth, as detailed in the Book of Genesis, provides a shorter span of time within which the earth as we know it today might have developed. There is a key within Genesis that allows the extent of that time to be determined. In the 17th Century an Irish Archbishop, James Ussher, sat down at his desk and calculated the age of the earth. He reasoned that one might do so by counting the generations of men as given in the Bible, back through such events as the capture of Jericho and Noah’s flood, to Adam and Eve in the Garden of Eden. By this method he derived 4004 BC as the year of Creation. This date was soon refined by Lightfoot at Oxford to 9 AM October 26, 4004.
The Ussher/Lightfoot calculus would only give the planet six thousand years from the creation to the present time within which to evolve. Such restraints would seem to favor cataclysmic events in the not too distant past, followed by a relatively static earth. Such a scheme was pushed forward in the 18th Century by Professor Abraham Gottlob Werner of the School of Mining in Freiburg, Germany. According to Werner, the rocky skeleton of our landscape had been precipitated out of a gigantic world ocean some time during the past six thousand years. This ocean was identified as having been identical with Noah’s flood, and hence Werner’s explanation was consonant with events as described in the Book of Genesis. He envisioned us as living in a young world, the geological features of which had been formed in the not too distant past and which had not changed significantly since. Certain features of Werner’s construct seem consistent with the facts. For instance, as much as 70% of the earth’s rocks are sedimentary. Sedimentary rocks must originally have been laid down horizontally (if they had been formed as sediment), the older layers under those laid down more recently. But the Werner theory suffered from inconsistencies with the facts. How to account for the wild disorder found in many rock formations? In nature, rocks are frequently upended, distorted, folded and discontinuous. How to understand igneous rock types such as basalt and granite, which seem to occur not in layers but rather to have intruded between and around layers of sedimentary rock? Where did these rock types come from? Additionally, Werner never explained what had happened to the water that must have been abundant enough in the great flood to cover the earth, including the highest mountains of the Alps. Late in the 18th Century Werner’s Neptunism was ripe for a challenge.
The challenge to Abraham Werner and to all other geotheorists who confined their schemes of formation of the earth within the traditional six thousand year boundaries came from a Scottish country gentleman, James Hutton. It’s curious that Hutton is not well known, while other pioneers of modern science such as Charles Darwin and Louis Pasteur are household names. Perhaps Hutton’s obscurity reflects the fact that the path to geologic understanding is strewn with rocks and boulders, while knowledge of evolution and infectious disease is more accessible.
James Hutton was born in Edinburgh in 1726. He initially was drawn to the study of
medicine and received a medical degree from the University of Leyden in
1749. By that time, however, his
interests had changed. He inherited a
farm in the Scottish borders near Berwick and spent the years 1754 to 1767 managing his property. He also, in those fruitful years, looked at
the geological formations in the landscape around him and came to certain
conclusions. In the first place he saw
no need for extraordinary mechanisms such as floods to account for its rock
formations. Instead he could explain
features of the landscape by the operation of mechanisms such as weathering and
erosion coupled with upheavals of the earth resulting from heat from within the planet. Certain igneous rocks, such as basalt and granite, the origin
of which Werner had ascribed to
deposition from the world ocean, Hutton found to be products of
extrusions of hot magma from within the earth.
Hutton kept looking for
features of the Scottish landscape awhich would prove his theory of the
formation of the earth. He found one of
the most striking of these at Siccar Point.
This point of land sticks out into the North Sea about thirty miles
south-east of Edinburgh. It consists of
vertical pillars of a sedimentary rock, greywacke, surmounted by horizontal
layers of a younger sedimentary rock, red sandstone. Hutton could only believe that the greywacke
had originally been laid down in horizontal layers and that it had been shifted
into the vertical by upheavals in the earth, following which layers of sand had
been deposited on top of the greywacke and had slowly hardened into red
sandstone. This process must have taken
long stretches of time, far more than would have been available for it in the
traditional scheme of Archbishop Ussher and Professor Werner. According to Hutton the earth should be
thought of as a dynamic system
functioning over “immeasurable time”.
At the core of Hutton’s thought was the notion that the same mechanisms
which govern the earth today have been present and functioning throughout its
history, and that our earth is the product of their activity. This view is called ‘uniformitarianism’,
since the development of the earth has been
uniform and not the product of extraordinary events.
James Hutton was a member of
the Scottish Enlightenment, a remarkable group of men centered in Edinburgh,
Scotland in the second half of the Eghteenth Century. They included David Hume, the philosopher,
Adam Smith, author of The Wealth of Nationa, Hutton’s close friend, the
chemist Joseph Black, discoverer of carbon dioxide, and James Watt, inventor of
the steam engine and one of the authors of the industrial revolution. These people looked critically at the world
around them and used the process of reason to interpret what they saw. The result was an upheaval in thinking and
the beginning of the modern world. On
March 7, 1785 Hutton’s ideas about the formation of the earth were presented
before the Royal Society of Edinburgh.
This lecture, subsequently published in a 95 page report in the
Transactions of the Society, established Hutton as one of the founders of
contemporary geology.
Hutton’s
concept of the dynamic earth was not immediately accepted by the people in
Europe interested in such matters, but it was not totally rejected either. Members of his immediate circle kept his
ideas alive. Then in 1830 a gifted
communicator, Charles Lyell, wrote one of the seminal books of the 19th
Century, The Principles of Geology, in which Hutton’s uniformitarianism
was explained in a convincing manner.
When Charles Darwin set off on his trip around the world in the Beagle
he took a copy of Lyell’s book with him.
For the theory of evolution to work an immense span of time was necessary. This time was provided by Hutton’s theory, as
transmitted by Lyell.
The Field Museum’s “Evolving
Planet” is based on Hutton’s concept of
a dynamic earth reinventing itself over periods of time that are so large as to be difficult for us to
comprehend. However, Hutton’s work by
itself was not enough to allow us to uncover the various stages through which
the earth passed from its first formation to the present time. It required the work of a number of able men
in Europe in the last years of the Eighteenth Century and first half of the
Nineteenth Century to create for us the geologic time scale as we know it
today. Such a time scale would show the
periods in earth’s history, the way the earth was shaped during those periods,
and the animal and plant life associated with them. You will notice that these men were able to
develop their theories of the formation of our planet because Hutton had freed
them from the old rigid boundaries of time.
Of these men who gave us the tools with which the geologic time scale
was created one of the most notable was
an Englishman, William Smith.
William Smith was born in
Oxfordshire in 1769. As a young man he
learned the art of land surveying through an informal apprenticeship. In the late 1790’s work on an estate in the
Somerset coal fields gave him experience
in surveying rock outcroppings. This was
the era of canal building. Canals were
necessary in England to move coal from mines where it was dug to the cities
where it was used. Smith as resident
engineer of the Smithfield Coal Canal Company was commissioned to survey the
line for two new canals and to supervise their construction. As the earth was removed with pick and
shovel in straight lines over
substantial distances in canal construction, the underlying rock strata were
revealed. Smith found these rock strata
to be rich in fossils. The fossils of
lower (and therefore older) strata were those of simpler organisms. Those of higher (and more recently deposited)
strata contained evidence of more complex organisms. Over distance, the fossils contained in a
single stratum of rock were found to identify that stratum. Early in 1796 Smith had written a brief
private note on the conclusions he had reached in his work on the canal.
He mentioned “the wonderful order and regularity with which nature had disposed of these singular productions [i.e.
fossils] and assigned to each class its peculiar stratum.” These “characteristic fossils” served to
identify a stratum. As time went on and
Smith traveled more widely in England, he was able to show that strata of rock
widely separated in distance could be connected one to the other by similarity
of contained fossils. This concept of
“characteristic fossils” was dependent, of course, on the notion that certain
life forms evolved, spread widely, and then died out during a defined period of
time. In 1815 Smith published a
“Geologic Map of England, Wales and Part of Scotland”, a rigorous treatment of
geologic information. It showed
understanding of the basic geological principles of original horizontality,
superposition (the younger rock over the older) and faunal succession. This latter term meant that fossils of life
generated more recently in time would be contained in younger rock. With these principles it became possible to
correlate strata from widely separated areas.
It also became possible to name the periods of geologic history, since
each stratum of rock with its characteristic fossils was connected with a
defined period in the development of the earth. The entire exhibit “Evolving Planet” is
based on the insights of James Hutton, William Smith and their contemporaries. They provided the tools with which the puzzle
of geologic time could be solved.
As we enter “Evolving
Planet” we are surrounded by the world of the Precambrian. This is the eon in the earth’s history
between its formation around 4.5 billion years ago from material orbiting the
sun and the explosion of
multicellular life during the succeeding
period, the Cambrian, which started 543 million years ago. The ocean of the
Precambrian is greenish in tint. Active volcanoes spread lava on the
land. The atmosphere is rich in ammonia
and methane, neither of which, of course, are visible as such in this
exhibit. The change to our
oxygen-rich atmosphere has not yet occurred.
. Although the Precambrian
occupies almost nine tenths of the earth’s time span remarkably little is known
about it. Precambrian rocks are hard to
find. They either have been eroded with
the passage of time or lie buried below layers of more recently deposited
materials. There are some evidences of life from as long ago as 3.4 billion
years. Well preserved bacteria that old
have been found in Western Australia.
The origin of that life is unknown.
Possible mechanisms of life formation are shown in the Field’s
Precambrian section.
As we pass out of the
Precambrian we come to the periods in which the life forms, the structure of
the continents and the rock formations are much better understood. Technically the whole span of geologic time,
from the end of the Precambrian to the present, is known as the
Phanerozoic. The bulk of the Field’s
exhibit “Evolving Planet” is composed of materials and illustrations from the
Phanerozoic. The Phanerozoic in turn is
divided into periods, starting with the Cambrian, which commenced 543 million
years ago. As we walk through the
Field’s “Evolving Planet” painted posts tell us which period we are in. Should we bother to concern ourselves with
these periods? Knowledge of them will
not help us much in our daily lives.
However they are entitled to a certain amount of respect. In the first place the time contained within
each of these periods is long, ranging from the Silurian, which lasted only 26
million years, to the Cretacious, which persisted for 79 million. Secondly, knowledge of them and of their unique
characteristics allows us to orient ourselves in the world from which we
ourselves have arisen. And finally, they
are the signposts which let us know where we are as we walk through “Evolving
Planet”. So here goes. Starting with the oldest, the Cambrian, which
lasted 53 million years, between 543 million years ago and 490 million years
ago, we read:
Cambrian, Ordovician,
Silurian, Devonian, Carboniferous, Permian,
Then a break for a massive extinction at the end of
the Permian
Then the age of the dinosaurs: Triassic, Jurassic, and Cretacious
Then another massive extinction, when the dinosaurs
disappeared, leaving as their only descendants the robins, chickadees, humming
birds and other bird friends that brighten our lives.
Then the 65 million years of the age of mammals,
ending with us.
You say, “I can’t cope with all these names.” I say, “Perhaps not, but let’s try. Let’s use a mnemonic, a device left over from
medical school.”
Let’s try C O S D - C P: “Cats obsess some dogs, cruel pooches”. Cats (Cambrian), obsess (Ordovician), some
(Silurian), dogs (Devonian) - cruel (Carboniferous), pooches (Permain).
That takes care of the six periods of the Paleozoic. The
three periods belonging to the Mesozoic, the age of dinosaurs, are easy:
Triassic, Jurassic (in the
middle, from the movie), and Cretacious.
After the Cretacious we are off and running in the age of mammals.
Each
of these periods was associated with changes in the continents due to plate
techtonics and with increasingly complex life forms. These planetary changes and developing life
forms appear in the exhibits of “evolving planet”. In the Cambrian period early life forms exploded in the seas,
including strage looking animals which have no modern equivalent. By the Ordovician trilobites were widespread
and green algae had appeared. The Silurian was a period of great
development. Big bony fish with movable
jaws dominated the ocean. Vascular
plants appeared on land as did the first terrestrial animals. In the Devonian fish developed legs and began
to walk on the earth. In the ocean giant
armored placoderms, fish with plates over their heads and upper body, appeared
in the ocean, along with the ancestors of sharks.
These geologic periods were
not identified in an orderly fashion, but rather randomly, as strata of rock
and their fossil content were examined, and as findings in one geographic
location were compared with those in others.
As one might expect, the first period to be clearly described was by the
commercially minded British. In 1822
William Conybeare and William Phillips coined the term “Carboniferous” or
coal-bearing to describe a succession of rocks rich in coal in north-central
England. In the Field Museum’s exhibit
the world of the Carboniferous appears as a hot, moist forest of large trees
and large insects. As those trees
fell, rotted, and were buried under pressure, seams of coal were formed. The Carboniferous lies in the Paleozoic
era, before the mass extinction at the end of the Permian.
The Carboniferous was followed
by the Permian, named after the town of Perm in Russiaa, where the stratum of
rock from that geologic period was originally identified. During the Permain a strange animal, the
synapsid, showed up. You will see
stunning examples at the Field. These were
the large beasts with huge fins
sprouting from their backs. The purpose
of these fan-like structures is unknown.
Did they dissipate heat? Collect heat? Attract mates? Frighten potential
adversaries? Perhaps all of the
above. In any event, they and the
animals to which they were attached did not survive the Permian extinction, and
no successors are alive today.
A Frenchman, D’Omalius,
found a common sequence of soft limestones, greensands, and related marls along
the coastal regions bordering the North Sea and the Baltic and named it the
Terrain Cretace. Again those pesky
English came along, in 1822 renamed
D’Omalius’ period the “Cretaceous”, and so it remains today. The Cretaceous was the last time span within
which the dinosaurs flourished. The
Field’s great Tyrannosaurus skeleton nicknamed “Sue” came from a beast which
lived during the Cretaceous. At the end
of the Cretaceous something happened – perhaps the arrival of a giant meteor -,
and the dinosaurs vanished. The Cretaceous is prominent once more at the Field
Museum (along with the preceding Triassic and Jurassic periods) in the hall of dinosaurs, the centerpiece of “Evolving Planet”. There you can see Herrerasaurus,
Ankylosaurus, Stegosaurus, Apatosaurus,
and the other members of the crew of Jurassic Park – and of Triassic Park and
of Cretaceous Park. They are an
impressive lot.
When the dinosaurs vanished
the mammals crept out of their holes and began to evolve. They were given 65 million years in which to
perform. During this period the animals
with which we are familiar, the horses, foxes, deer, porcupines, and all the other mammalian
inhabitants of the animal world took shape.
Late in this era, about four million years ago, the first humanoid,
Australopithicus afarensis, began to walk around the savannas of Africa on two
hind legs. He (or she) is not an
important player in this exhibit, but he is there for completeness as he
struggles to separate himself from the apes, his cousins.
The last 1.8 million years
before the present was mostly occupied by the Pleistocene, the time of the
great glaciers. The last great hall of
“Evolving Planet” is home to the animals of the Pleistocene. Most of them are big, even huge. Large size (or a large volume to surface
ration) was an asset during a time of the glaciers, which marked the
Pleistocene. In this hall we meet a
mammoth, a mastodon, saber-toothed tigers, a ground sloth perhaps 18 feet tall
and a short-faced bear perhaps 10 feet from paw to snout. The Irish Elk, which sported enormous
antlers, is there as well. How our
ancestors made it alive through the ranks of these competitors remains a
mystery., However, we are still here and
they are gone, which is too bad in many ways, since they were impressive.
Now you will notice that we
have casually thrown around dates for
events in the world’s past. For example
we have implied that the earth itself coalesced from material in orbit around
the sun roughly 4.5 billion years ago and have stated that the explosion of
life which ended the Precambrian occurred 543 million years ago. These are absolute dates in the geologic time
scale. They were not available to the
pioneers in the field of geochronology in the Eighteenth and Nineteenth
Centuries. These men dealt only with the
relative time scale. They determined
that the fossils associated with the Ordovician were older than those of the
Silurian, and that these latter preceded in time those of the Devonian. They did not pretend to know when each of
these periods started and stopped, in other words the absolute time scale.
Efforts, of course, were made to determine at
least the age of the earth itself. The
notion that salt had leached out of the land into the ocean at a constant rate
was first proposed in 1691 and reinvestigated by John Joly in Ireland in
1899. He measured the salt in the ocean
and proposed that these oceans had been
created between 80 and 90 million years ago.
Buffon, in France in 1775, measured the rate of cooling of iron balls of
various sizes and extrapolated his findings to the size of the earth. He concluded that the earth was 74, 832 years
old. The British physicist Lord Kelvin
applied thermodynamic principles to the supposed cooling of the earth and
concluded, also in 1899, that the earth was between 20 and 40 million years
old. Other investigators looked at rates
of sediment accumulation with equally dicey results.
Finally a satisfactory
method of determining the age of rocks came from the discovery of radioactivity
by Henri Becquerel in France 1896 and the subsequent finding that radioactive
elements decay at a constant rate over time.
This latter observation was key to our understanding of absolute
geologic time. The notion that we might
use studies of radioactivity made today as though they had been valid four
billion years ago goes back to James Hutton’s principle “that events occurring
today at the earth’s surface have their counterparts in processes occurring in
geologic time”. Thus if one could measure
the content of a radioactive element in a rock and simultaneously the amount of
a daughter element and if one knew the
rate of decay of the element, then the age of the rock could be
determined. In 1905 the British
physicist John Willliam Strutt succeeded in analyzing the helium content of a
radium-containing rock. He determined
that its age was 2 billion years. In
1911 one of Strutt’s students, Arthur Holmes, investigated rocks from several
Paleozoic geologic periods as well as those from the Precambrian. As a result of his work and that of his
successors, the relative geologic time scale was numerically quantified and
converted into the absolute time scale
of geologic periods and events which we use today.
It’s hard to conceptualize
the time spans of the absolute time scale of earthly existence. Perhaps one way to approach this challenge is
to take a familiar time span and see how it compares with geologic time. The Norman Conquest of England took place
about a thousand years ago. The time
between now and then is our handhold on a thousand years. A
million years, the time from the present
to the middle of the Pleistocene, is a thousand times our base thousand
years. 65 million years, the time to
the dinosaur die-off, is 65 times
that. In other words the time back to
the Norman Conquest is 1/65,000 of the
time to the last Tyrannosaurus, a mere blip.
Our base thousand years is
slightly less than 1/500,000 th of the time back to the Precambrian, when
multicellular life first appeared on earth, a blip of a blip. And, of course, most of earth’s history,
9/10ths of it, took place during the long years of the Precambrian, when the
only living witnesses to events on earth
were a few bacteria.
Knowledge
of the age of the planet earth, knowledge of the life forms which have flourished
during the different geologic periods, knowledge of the first appearance of
man-like creatures perhaps four million years ago and of the emergence of man from his base in Africa between 1.6 and 2
million years ago should give us some insight into our position in the
evolutionary scale. It should emphasize
to us our responsibility for the earth, of which we are the very recently
arrived remaindermen, and our kinship with and responsibility for the
other creatures on this fragile earth.
All these considerations are brought to mind
during a walk through the Field Museum’s “Evolving Planet”
Robert
W. Carton
27 May 2007
