recap

518

24.2 recap

Physical conditions on Earth have changed dramatically over time. Changes in Earth’s climate and sea levels have had major effects on biological evolution. Continental drift, volcanic eruptions, and large meteorite strikes have contributed to major climate changes during Earth’s history, and many of these climate shifts have resulted in mass extinction events. Changes in atmospheric concentrations of O2 have also influenced the evolution of life, and the evolution of life has, in turn, affected the physical nature of the planet.

learning outcomes

You should be able to:

  • Place important events in biological history onto a time line of Earth’s history.

  • Characterize the relative impact of weather events and climate changes on Earth’s physical conditions.

  • Explain how the production of atmospheric oxygen has affected Earth and its living organisms.

  • Analyze the connections between a specific geological event or environmental change (e.g., fluctuations in ocean currents or sea levels, volcanic eruptions, meteorite strikes) and an evolutionary trend among living organisms.

Question 1

Explain why an occasional major winter blizzard is irrelevant to discussions of global climate warming.

Climate refers to long-term average expectations of weather at a given location; weather refers to the daily events. Even if the long-term expectations of climate are of gradual warming, we still expect occasional winter blizzards at a given location away from the tropics. A long-term increase in the average temperature of 1°C represents a major climate change, but we would still expect a wide range of temperature conditions throughout the year at a given location.

Question 2

Describe changes in the environment that may have favored the evolution of groups of multicellular organisms near the end of the Precambrian.

Several major environmental changes occurred near the end of the Precambrian. This was the end of the “snowball Earth” period, when life was likely confined to a few relatively warm areas, such as hot springs and thermal vents. Warming of Earth resulted in rapid diversification of life. There was also a rapid accumulation of oxygen in the atmosphere. Multicellular organisms have higher metabolic requirements than do single-celled organisms. As O2 accumulated in Earth’s atmosphere from the earliest photosynthetic organisms, it was initially toxic to the single-celled, anaerobic organisms that existed on early Earth. Some species evolved to tolerate and then use O2 for aerobic respiration, which allowed the evolution of large colonies of organisms. Eventually, cooperation among the different individuals in the multicellular colonies led to the evolution of integrated, multicellular organisms.

The many dramatic physical events in Earth’s history have influenced the nature and timing of evolutionary changes among Earth’s living organisms. We now will look more closely at some of the major events that characterize the history of life on Earth.