Lecture 23: April 29

• Human evolution and climate change
• Diversity over time re-visited

• Gould, S. J., 1994. The evolution of Life on Earth. Scietific American, October 1994, p. 63-69; the text without figures is available on the web: 'Contingency of Evolution'. Reprinted 2004.
• Benton, M. J., 2001. Biodiversity on land and in the sea. Geological Journal, v. 36, p. 211-230. A .pdf file of this paper can be downloaded from this site.
• Lewontin, R. C., 19993. Biology as ideology, Chapter 5: Science as Social Action. p. 107-123.

Over the last decade several different theories linking human evolution with climate change have been proposed.

1. Savannah hypothesis
2. Turnover-pulse hypothesis
3. Variability-selectivity hypothesis
4. 'Bottleneck effects'

1. Savanna hypothesis:

• Vegetation change from more forested conditions to more open vegetation => hominids move out of trees, become bipedal => don't use hands for walking and thus free them for tool use => brains develop.
• Intuitively attractive, but did humans become bipedal at/after climate change?
• Early bipeds (at 5-6 Ma) living in wooded areas (from co-occurring fossils, sedimentary evidence) seen as evidence against this hypothesis

2. Turnover-pulse hypothesis

• Broad-based faunal turnovers (not just human evolution) linked to climate change (3.0 to 2.5 Ma) => causes changes in the whole African ecosystem => coordinated evolution of hominids, antelopes, and small mammals (rodents)
• Magnitude, timing and rapidity of the evolutionary changes are challenged. Various investigators doubt that such pulses occurred at right time for human evolution

 Note that according to this theory relatively long periods of stasis (little change) alternate with much shorter periods of faunal turnover. We thus should see coeval changes in species composition within a whol assemblage (a large part of the assemblage changes). The rates of evolution (number of species originating or becoming extinction per time unit) must be significantly higher during the times of turnover than during the times of stasis.

3. Variability-selectivity hypothesis

• Environmental oscillations (such as alternations between glacial and interglacial periods) cause evolution: repeated changes in landscape and vegetation => break-up of continuous habitats => separation of populations => allopatric speciation.
• How to test? Evolution could happen at any time, during and after climate change. Genetic patterns in living descendants?

Note that according to this theory overall rates of evolution are high during periods of strong and relatively rapid climate change, such as the Plio-Pleistocene Ice Ages, when changes occurred on time scales varying between ~40 and 100 kyr. There is no requirement that different species within an assemblage changes at the same time; just that the overall evolutionary rates are high during intervals of climatic instability because of the common creation of marginal areas for a species, enhancing the possibility of allopatric speciation.

4. 'Bottleneck effects'

• At least once, possibly several times, hominoid populations went through a 'genetic bottleneck', i.e., almost went extinct (~ 1000 of individuals)
• Effect on long-term evolution? Loss of alleles, less variability of gene pool
• Rapid evolution in small population
• IF human-type intellect evolved at 70 kyr, reinforcement for theory because at ~75-71 kyr ago climate cooled globally; small population size could have been cause of rapid evolution.
• At 75-71 kyr: orbital peridicity predicted fairly mild ice age, reinforced by huge volcanic eruption of explosive type: Mt. Toba (Sumatra, Indonesia). Interesting: if indeed 'modern humans' (use of symbols) date back until about 70-77 kyr....

These will all have to await time control of events in human evolution.

• Exaptation: our brains did not evolve in order to work on particle physics,brain surgery, or play the piano or to think symbolically. Possibly, once our brains reached a certain threshold of complexity, such functions just did become possible.
• Neoteny: change in regulatory genes: we are born as fetuses, never grow up to true adults; our brains remain as those of juvenile apes.

What are the long-term trends in evolution?

From less to more complex, from simple to more diverse.

But HOW did this happen?

• What are the rules that govern biodiversity? [In which environments is biodiversity  presently highest? How has biodiversity changed over geological time?]
• Why are major evolutionary innovations unevenly distributed in time and space? [In  particular: mass extinctions. How are they triggered? What happens during a mass  extinction: which groups are most vulnerable? Is this the same for all mass extinctions?]
• How does the biosphere respond to environmental perturbations at global and regional  scales? [Mass extinctions again, but also smaller scale environmental changes. What happens during ice ages? Do species move or become extinct or evolve?]
• How have biological systems influenced the physical and chemical nature of the Earth's surface and vice versa? {there has been a growing awareness that biota, and not only human biota, have evolved together with the Earth).

•  "Keeping up with the neighbours", also called the "Red Queen model" (Alice in Wonderland, to whom the Red Quenn said: "Here we always have to keep running to stay in the same place"), or the "arms race model". The cause of evolution is continuing  competitionas well as cooperation between species and organisms. The results are that all organisms that are now alive are "better" than earlier living, extinct ones: a recent carnivore could easily  out-compete a herbivore of 15 million years ago. There would be vigorous evolution,  even in the absence of environmental change. This idea sounds very attractive:  everything works out for the best, we are the best that ever lived.
• "Just plain good luck", or "be in the right place at the right time" model. The cause of  evolution consists of mass extinctions, killing indiscriminately, regardless of how well adapted organisms are, leaving many niches open for new organisms to invade and exploit. The results are that we have no clue whether a mammal would out-compete a dinosaur in a similar niche in life. The biosphere gets 'reset' with every catastrophe; there would be no evolution without environmental change. This idea does not sound that attractive: we are not the final great result of billions of years of evolution, but just of the luck of the draw.

Few scientists (if any) adhere to either extreme view, but there is a very lively debate on which of these two possibilities is dominant in determining long-term evolution.

• Difficult to test: in complex organisms there MUST be a time lag between evolutionary change and the driving climate change. Example: evolution of dolphins and whales and global cooling (increasing oceanic productivity). But how much of a time lag? And if there is a time lag, then how do we argue for causality?
• What ARE  external factors? Environment and biota are mutually dependent, even on a global scale, through biogeochemical cycles.
  • Oxygen in the atmosphere (now about 20% of all atmosphere) is produced by biota. 
  • Ozone in the stratosphere is derived from the oxygen produced by biota (necessary for life on land, to filter UV light)
  • Carbon dioxide in the atmosphere and oceans is used by organisms; the carbon is stored  in organic matter (fossil fuel) and limestone, and thus can no longer influence climate.  Much environmental change may be driven by organisms (e. g., oxygen in atmosphere; CO₂ levels and thus cooling/warming).

Diversity increased. Note that presently biodiversity on land is much higher than in the sea (as to number of species). This can have been so only since the evolution of forests (Devonian-Carboniferous).

Land record not as bad as commonly said; sedimentation rates high. Various measures of 'completeness' not that different between land and sea fossil records. Comparison between cladograms and stratigraphic rceord not that different between land-sea either.

Biota on land: diversity increase (family/genus/species level) appears to be exponential, rather than logistic (which would mean that there would be a maximum number of species, a species 'carrying capacity', for the world).

In the oceans: not clear. It has always been argued that the diversity of oceanic families reflect equilibrium during the Paleozoic (see figures 8.19, 8.20 in text book; lecture 12), but was that so? It may also be interpreted as damped exponential: note that the slope of species increase was steeper after mass extinctions.

Maybe the apparent equilibrium during the Paleozoic (for oceanic life) is there only in the record for families, not in that for lower taxonomic categories (see figures below, from Benton, M. J., 2001. Biodiversity on land and in the sea. Geological Journal, v. 36, p. 211-230. A .pdf file of this paper can be downloaded from this site).

Maybe life on earth is not in an equilibrium situation: diversity was still increasing rapidly when humans evolved (e.g., Teleost fish, flowering plants, ruminating herbivores). IF life on Earth was diversifying exponentially (at the species level), with that exponential increase only slightly interrupted by mass extinctions (but these were very severe at the species level, e.g. , >90% of species estimated extinct during end Permian extinction), then it seems that interaction between species ('Red Queen') must have been a very important cause of evolution during the Phanerozoic.

• NO if progress is defined as 'evolution towards humans'; there are too many organisms  that do NOT fit on an 'amoeba-human trajectory' (e.g., all protostomes; plants)
• YES, if meant as 'increased complexity': from single-celled prokaryotes to extremely diverse, intricately entwined, complex living forms
• MAYBE YES, if meant as 'better engineered', and almost certainly YES if progress means"a more energetic life style".
• BUT maybe there were environmental controls on the development of such an energetic life style at earlier times (e.g., not enough oxygen in  the oceans).

Example: evolution of horse could be seen as better and better adapted to grazing (grass) rather than browsing (leaves). BUT that is only "better" if the environment does indeed change from more forested to more grass lands...