READING PASSAGE 3
You should spend about 20 minutes on Questions 29-40 which are based on Reading Passage 3 below.
As Darwin discovered his evolution theory, the earliest known fossils were left in rocks which he called the Silurian age. Older rocks seemed to contain no fossils. The apparently sudden appearance of subtle animals like trilobites was not inconsistent with Darwin’s thoughts of gradual evolution. “If my theory will be true, it is unquestionable that before the lowest Silurian stratum was deposited … the world swarmed with living creatures. To the question why we do not find records of these vast primordial periods, I can give no satisfactory answer,” Darwin wrote in the first edition of On the Origin of Species. His puzzle is known as Darwin’s dilemma.
Of course, we have discovered a lot of fossils from the earliest periods. Rocks of 3.8 billion years old have signs of life, and the first recognizable bacteria come out in rocks of 3.5 billion years old. During the Ediacaran, approximately one billion years ago, multi-cellular plants with red and green algae appeared and approximately 575 million years ago was found in the first multi-cellular animals.
Even so, there are many perplexing questions. Why did animals evolve so late in the day? And why did the ancestors of modern animals apparently evolve in a geological blink of an eye during the early Cambrian period between 542 and 520 million years ago? Recently, a series of discoveries could help to explain these long-lasting mysteries. These discoveries suggest that the earliest animals evolved much earlier than we thought, perhaps over 850 million years ago. However, the really extraordinary part is that these early animals may have completely changed the planet, paving the way for the larger and more complex animals to follow them.
Several aspects of the biggest discoveries have come from an ancient seabed in China, called the Doushantuo Formation, where unusual conditions conserved some extraordinary fossils. During the last part of the Ediacaran period, layers between 550 and 580 million years old include tiny spheres made of from one to dozens of different cells – just like animals’ first embryos. A couple of things have suggested that they are the property of giant bacteria, but a series of studies over the past decade have left little doubt that they are really animal embryos.
Leiming Yin, a researcher at the Nanjing Institute of Geology and Paleontology in China, reported discovering embryos encased inside hard, spiky shells unlike anything produced by bacteria in 2007. Furthermore, evidence of shells that apart from the deficiency of conserved embryos on the inside are identical can be seen in rocks as old as 632 million years – the appearance of the Ediacaran period – suggesting that the animal embryos themselves go back this far.
Other more tentative discoveries push the appearance of animals back even further. Roger Summons, a researcher in the Massachusetts Institute of Technology, and his colleague Gordon Love studied brownish, oily sandstone cores drilled from 4 kilometres below the desert of Oman. The oily remains of dead organisms drifted down to the depths of ancient oceans, where they decomposed slowly because of the lack of oxygen. No visible fossils are present but within that oil are molecular fossils – chemicals taken from the ancient organisms. In layers that are 635 to 713 million years old, Summons and Love discovered 24-isopropylcholestane (24-IPC), a stable form of a kind of cholesterol that these days are only discovered in the cell membranes of certain sponges. “The sponge biomass must have been so substantial,” says Love, now at the University of California, Riverside. “They were ecologically outstanding.”
Fuel of Life
With the oceans changed, the stage was finally set for the evolution of more complicated body forms. The idea that increasing oxygen levels played a major role in the explosion of life during the Cambrian period is far from new, but most of the researchers attribute the increase in oceanic oxygen to the increase in the atmosphere. If Butterfield is right, it was basically because of animals taking over from bacteria. “These geochemical signatures [of oxygenation] are not causing the evolution of animals,” he insists. “They’re consequences of the dawn of animals.”
“He is right,” says Brasier. In fact, he thinks the link between complex life and the transformation of the planet runs even deeper. In Darwin’s Lost World, a book published earlier this year, Brasier suggests that the improved burial of carbon resulting from the rising of large cells and groups of cells – perhaps with plants like seaweed – sucked carbon dioxide out of the atmosphere, setting off the series of ice ages that aided the first animals to wrestle for control of the oceans with bacteria. “Rather than being the cause of animal evolution, the ice ages may well have been the response to it,” he says.
Look at the following statements and the list of researchers below.
Match each statement with the correct researcher (s), A-E.
Write the correct letter, A-E, in boxes 29-33 on your answer sheet.
NB You may use any letter more than once.
29 studied brownish, oily sandstone cores.
30 announced embryos on the inside surrounded by hard, spiky shells.
31 claimed that the expanded burial of carbon resulted in the series of ice ages.
32 wrote in the first edition of On the Origin of Species.
33 discovered 24-isopropylcholestane
|List of Researchers|
|Summons and Love|
Answer the questions below.
Choose NO MORE THAN THREE WORDS from the passage for each answer.
Write your answers in boxes 34-36 on your answer sheet.
34 What is an ancient seabed in China, conserving some weird fossils?
35 What made organisms decompose in the depths of ancient oceans?
36 What was written by Brasier to swell burial of carbon resulting from the rise of large cells and groups of cells?
Complete the summary below.
Choose NO MORE THAN FOUR WORDS from the passage for each answer.
Write your answers in boxes 37-40 on your answer sheet.
Fuel of Life
From the oceans fluctuated, 37 _______ of increasing levels played a vital part in the increase of oceanic oxygen in the atmosphere. Actually, Brasier considers the connection of 38 _______ and 39 _______ goes deeper. According to Darwin’s Lost World, he claims that carbon burial was getting more inhaled 40 _______ outside of the atmosphere, caused the series of ice ages that was supported with the first organism generated from bacteria.
|35||Lack of oxygen|
|36||Darwin’s Lost World|
|39||transformation of the planet|