The modern ecosystem includes both oxygen producers and oxygen consumers that, as we have seen in the previous chapters, were added to a primordial anaerobic world. But how old is this modern ecosystem? Is it possible to trace its history?
We can answer both of these questions, by looking at three lines of evidence:
- The fossil record of cyanobacteria, the earliest-evolved "complete aerobes"
- Minerals found in ancient rocks that can tell us how much oxygen was in the atmosphere
- Chemical signals (isotopes) preserved in sedimented organic matter that show whether photosynthesis was going on
1 - EVIDENCE FROM THE OLDEST FOSSILS
The oldest fossils known are the Apex fossils, dated at 3.465±5 billion years thanks to volcanic beds that entomb them.
Try to focus on the following questions, and the relative answers that can be found in the text of Chapter 6:
- Are these fossils real? Why, or why not?
- Would these fossils be easily preserved?
- Based on the record that has been found, can we say that several kinds of microbes were thriving on Earth at the time indicated by the Apex fossils?
- Are these fossils the oldest?
- What about the Isua graphite, dated at 3.8 billion years? (Note that the Isua graphite is NOT the oldest rock ever, but simply the oldest rock containing graphite, which might derive from organic matter. The oldest rock ever found is a metamorphic rock, a gneiss, called the Acasta gneiss, from the Canadian Northwestern Territories, which has been dated at 3.96 billion years)
- Why should we look at the Moon for a clear answer?
- When did the meteorites cease to hit Earth, and how long before the Apex fossils?
- If life was so flourishing by the time of the Apex fossils, how did evolution advance so far, so fast?
- Gene xeroxing and evolutionary remodeling are the keys to life's early advance. What does that mean?
- Why is it that we can not compare the last 400 million years of evolution (from 400 million years ago to the present) with the first 400 million years (from 3.9 billion years ago to the the Apex fossils at 3.5)?
- What would you call the combination of traditional paleontology with geology and isotopic geochemistry?
2 - EVIDENCE FROM GEOLOGY
Of all microbes, only cyanobacteria can carry out photosynthesis.
sunlight
6H2O + 6CO2 ——> C6H12O6 + 6O2
Photosynthesis requires water (H2O) and carbon dioxide (CO2), and produces organic matter (C6H12O6) and oxygen (O2).
Do we have evidence for the presence of all these four chemical compounds in the Apex? Yes, we do.
This evidence is described in the pages of Cradle of Life, and you should be able to find by studying this chapter.
In particular, Chapter 6 deals with the recognition of the existence of oxygen gas in the atmosphere during the Precambrian. A strong indicator of this presence is given by rocks rich in oxidized iron known as the BIFs (Banded Iron Formations).
An image of the Banded Iron Formations from the Archean of Minnesota
- What are BIFs?
- What happens to iron in the ocean when oxygen is present?
- Where does the oxygen in the atmosphere mostly come from?
- Know the destiny of both Fe and O during three distinct time intervals:
- before 3.5 billion years ago
- between 3.5 and 2
- after 2 billion years ago
- Given the amount of oxygen required to make BIFs, what is the only possible source for it, given that there was basicly none at the beginning of Earth history?
3 - EVIDENCE FROM ISOTOPIC GEOCHEMISTRY
Review radioactive and stable isotopes: what are isotopes and what are the differences between the two kinds. It is a partial anticipation from chapter 10 on the textbook. Make the case of the three C isotopes (12C, 13C, 14C).
Because animals obtain their nourishment from autotrophs, autotrophs govern the isotopic distribution of 12C and 13C in the living world.
- Why do you have isotopic fractionation during photosynthesis?
- Is the organic matter produced during photosynthesis "lighter" or "heavier"?
- What about inorganic limestones?
- What is the average difference, in parts per thousand, between the 12C isotopic values in organic matter and in limestones?
- If this difference indicates photosynthesis, when did it start to appear in the fossil record? Do we see it in the Apex fossils?
- Can we see similar signals in chemosynthetic processes when we stuidy sulfur isotopes?
- Do we see signs of methanogenesis?
- How does fractionation of 12C differ in methanogenesis in comparison to photosynthesis?
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