Читать книгу Viruses: More Friends Than Foes (Revised Edition) - Karin Moelling - Страница 29

Early life created its own growth conditions. It did not adjust to the existing environment, but created the necessary conditions itself, especially oxygen. Light contributed (by photosynthesis) to the increase of oxygen, which displaced the then dominant methane. The oxygen concentration increased and became the basis of breathing of mammals. About 2.2 billion years ago the earth froze to ice. Yet life already existed. Where did it hide? Next to the stove, the hot hydrothermal submarine vents? Did they protect life from dying out? It is also conceivable that life started not in the warm but in the cold; early life may have existed in ice crystals with fluid passing through channels, and some molecules may have led to biomolecules such as RNA. This theory has some adherents, because RNA has been found within ice crystals with fluid streams inside. Model RNAs are even under investigation after storage in deep freezers in the laboratory — does it change, mutate or evolve? In the proximity of the black smokers, life progressed rapidly: 2 billion years ago there were oceans with bacteria and unicellular organisms, colonies of algae grew, and 600 million years ago sponges, jellyfish and worms developed; 500 million years ago there were hard shells, corals and teeth. Then there came the so-called Cambrian explosion, the dramatic increase in numbers of all forms of life. The first vertebrates appeared. 300 million years ago the mottled fish Coelacanth appeared, which was thought to have become extinct but was then discovered accidentally by a museum’s director in a fisherman’s net in South Africa in 1999. Its genome was sequenced, and this made it possible to reconstruct the transition from fins to legs. This is a conclusion drawn from the genes! 250 million years ago the earth consisted of a supercontinent, Pangea, and a first mass extinction occurred which makes me wonder whether it could happen again.

65 million years ago a catastrophe killed up to 90% of all living species. This was due to an asteroid, a large meteorite, as big as the Himalaya mountains, which hit the peninsula of Yucatan in Mexico: Samples from drilling in the bottom of oceans, as collected by the marine research center “MARE” in Bremen, supplied the proof of this collision. A 15-cm-broad dark layer represents fire and ashes. This layer was designated as the K-T layer, indicating the transition period between the Earth’s Cretaceous and Tertiary ages. This K-T extinction layer can be found all over the world. The sudden increase in amounts of the element iridium, termed the “iridium anomaly”, is also regarded as proof of the hit by an extraterrestrial carrier. The asteroid of Yucatan resulted in a tsunami, 100 m high; the sun was darkened by dust, and the dinosaurs became extinct. Their death opened up the chance for other living animals, small ones, similar to mice, which were omnivores and ate everything without preference. That helped them to survive. They could hide underneath the surface in protective holes. Finally, we arrived on the scene! Five million years ago monkeys developed. Chimpanzees are our closest relatives, with a genetic content that is 98.4% identical to ours. What distinguishes us from them? Not only a bigger brain, but also special — combinatorial — features of our genes has increased our complexity; this will be discussed later. 3.2 million years ago, Lucy lived in Eastern Africa, the oldest human ancestor. 1.7 million years ago, Homo erectus appeared, who came out of Africa. Perhaps he was able to make and use fire. Could he speak? He became extinct. Then, 200,000 years ago, Homo sapiens appeared, again out of Africa, and colonized Eurasia until 60,000 years ago. The Neanderthals lived from about 200,000 years ago until they died out 40,000 years ago. Did they die of hunger, pathogens, diseases, cold or climate changes? They left behind wall paintings in caves and some genes in our genomes. Here I dare to include a comment: I am not so surprised about this as some newspaper-columnists have been. Yet I do not believe reports that the Neanderthals suffered from depression which some humans still suffer from today. Who knows the sequence of a depression gene(s)?

12,000 years ago hunters and gatherers settled and became farmers. Then our ancestors started drinking milk, or at least the majority of them did. Some, like me in fact, have never adjusted to milk up to this very day. A mutation is required for its digestion. Milk protected humans against vitamin D deficiency — a selective advantage for those who live in darker Northern regions of the world. About 12,000 years ago the last ice age ended, and humans could walk on foot from Siberia to America. That was the beginning of our civilization.

Proximity between livestock and humans led to diseases transmitted from animals to humans, by the mechanism referred to as zoonosis. This remains until today the most frequent cause of infectious diseases: “bush meat” with monkey viruses ended up in the food chain of humans, causing HIV, and Ebola originates from fruit bats. During the past 2.5 million years there have been about 20 ice ages, and the warm intervals were more like intermediate periods between ice ages. One cause of these is the change of the orbit of the earth around the sun every 100,000 years. It makes it easy to remember the dates of the ice ages: 450,000, 350,000, 150,000 and 50,000 years ago. The last one ended about 12,000 years ago. The Earth’s axis turns in cycles of 24,000 years and by doing so influences our climate. The next change, with an ice age, will come in 39,000 years. Who thinks of ice ages — the opposite, global warming is our present discussion. There are no known models in the history of our planet to explain rapid changes in the weather — but since there have been ice ages, global warming must have occurred before.