1. Fossil pollen grains

2. Palynology
Palynology is the analysis of fossil pollen and is used to reconstruct long run changes in vegetation and climate.
Palynology

3. What is pollen?
Pollen grains are the sperm-carrying reproductive bodies of seed plants like conifers, cycads, and flowering plants.
Each of these grains has its very own unique shape depending on what plant it comes from, and their walls are made of a substance known as sporopollenin, which is very chemically stable and strong.
Pollen being an aeolian (wind-blown) sediment will accumulate on any undisturbed surface.

4. When pollen grains are washed or blown into bodies of water, their tough outer walls allow them to be preserved in sediment layers in the bottoms of ponds, lakes, or oceans.
Since pollen grains rarely rot, they are well preserved in the sediment layers that form at the bottom of a pond, lake or in blanket peat.
Because of their unique shapes, scientists can then take a core sample of the sediment layers and determine what kinds of plants were growing at the time the sediment was deposited.

5. A pollen count for a specific layer can be produced by observing the total amount of pollen categorized by type (shape) in a controlled sample of that layer.
Changes in plant frequency over time can be plotted through statistical analysis of pollen counts in the core.
Knowing which plants were present leads to an understanding of precipitation and temperature, and types of fauna present.

6. What can pollen tell us?
By analysing pollen from well-dated sediment cores, scientists can obtain records of changes in vegetation going back hundreds of thousands, and even millions of years.
Comparing trends in vegetation from the last few thousand years to recent trends in vegetation can also help scientists determine whether human activities have had significant impacts on ecosystems.
Jonathan Drori - Ted talk