The transition from dense forests to open savannas during the Miocene–Pliocene (about 23 to 2.6 million years ago) was a result of long-term climate change and tectonic activity that reshaped ecosystems across the globe. Here’s a breakdown of the key factors:
I.
Global Cooling and Drying Trends
1.
Decline in Global Temperatures
- After the early Miocene climatic optimum (~17–15 mya), the Earth began to cool gradually.
- This cooling was part of a long-term trend that began in the late Eocene and intensified during the Miocene and Pliocene.
- Cooler temperatures reduced evaporation, leading to less rainfall in many tropical and subtropical regions.
2.
Decreased Atmospheric CO₂
- CO₂ levels declined significantly during the Neogene (Miocene–Pliocene).
- Lower CO₂ impairs the efficiency of C₃ photosynthesis, used by many trees.
- In contrast, grasses with C₄ photosynthesis—more water-efficient and better adapted to low CO₂—thrived in the new conditions.
II.
Tectonic Uplift and Continental Rearrangement
1.
Formation of Mountain Ranges
- Major uplifts—such as the Himalayas, East African Rift, and Andes—altered global and regional climates by:
- Redirecting atmospheric circulation,
- Causing rain shadows (dry leeward areas),
- Cooling the land surface and influencing monsoon systems.
2.
Closure of Oceanic Gateways
- The closure of the Tethys Sea and formation of the Isthmus of Panama (~3 mya) disrupted ocean currents.
- This shifted heat distribution and intensified polar ice growth—reinforcing global cooling.
III.
Fire and Herbivory: The Grassland Feedback Loop
1.
Increased Fire Frequency
- Drier environments meant more frequent wildfires, which suppressed tree regeneration and favored fire-resistant grasses.
- Grasses grow from their base, not their tips, so they recover quickly after fire—unlike trees.
2.
Rise of Grazing Mammals
- Many herbivores evolved high-crowned teeth (hypsodonty) to graze on abrasive grasses.
- Their foraging further suppressed woody vegetation and expanded grasslands in a positive feedback loop.
IV.
Savanna as a Mosaic Habitat
- Forest fragmentation gave way to woodland–grassland mosaics, especially in East Africa and parts of Asia.
- These savannas provided new ecological pressures and opportunities:
- Greater visibility (changing predator-prey dynamics),
- Necessity for long-distance travel (favoring bipedalism in hominins),
- Open niches for generalists like early humans and their ancestors.
Summary Table: Why Forests Gave Way to Savannas
|
Cause |
Effect on Vegetation |
|
Global cooling |
Reduced rainfall, contracted humid forests |
|
Declining CO2 |
Disadvantaged trees (C3), favored grasses (C4) |
|
Tectonic uplifts |
Created dry zones, changed atmospheric flows |
|
Fire regime intensification |
Suppressed trees, promoted fire-adapted grasses |
|
Grazing mammal evolution |
Accelerated grassland expansion |
|
Oceanic current shifts |
Reinforced cooling and seasonal climates |
Conclusion:
The forest-to-savanna transition was not a singular event but a complex, gradual transformation driven by climate cooling, tectonics, atmospheric shifts, and ecological interactions. It reshaped the evolutionary path of many species—especially hominins, who adapted to bipedalism, tool use, and cooperation in these vast, open landscapes.
Would you like a visual timeline of this transition or a geographic map showing where major savannas formed during this period?
