Plant Science Portal 
 Plant Evolution Tour: Part XIV of XV

The Tertiary which lasted from 66.4 to 2.5 Ma was the time during which the angiosperms established dominance of the plant kingdom in many parts of the world. The most significant development over this period was the evolution of the grasses.

The monocotyledons evolved in the early Cretaceous (c. 110 Ma) but it is uncertain from what they evolved.

Palaeobotanists generally accept that there are two possibilities for the evolution of the monocotyledons; they either (1) evolved from ancestral dicotyledonous species after which time they lost the cambial activity of their stems and experienced a fusion of their cotyledons or (2) the ancestors of modern monocotyledons were slightly differentiated monocotyledons from which the dicotyledons later evolved (Dahlgren, Clifford & Yea 1985).

The evolution of species within the monocotyledon group (Liliopsida) has been recently assessed by scientists. By comparing the genetic similarity of different species from different families of monocotyledons, it is possible to gain an understanding of the evolution of the monocotyledons. 
Agapanthus praecox
Agapanthus praecox represents one of the older monocotyledon families.
One important event in the evolution of the monocotyledons occurred about 80 Ma which effectively provides a 'divide' between the newer monocotyledons and the older ones.

All plants contain the 5'-(TTTAGGG)n-3' DNA repeat sequence in the telomeres of their chromosomes except for one group of monocotyledons. Almost all of the monocotyledons that evolved after the split of the family Doryanthaceae from other monocotyledons about 80 Ma do not contain this otherwise common sequence (Adams, Hartmann et al., 2001). The presence or absence of this marker, along with other characteristics, provide a clue to the age of each monocotyledon family.

Therefore, families such as Agavaceae (Agaves), Liliaceae (Lilies), Amaryllidaceae (Amaryllis etc.) and Iridaceae (Irises) represent some of the 'older' monocotyledon families. The newest monocotyledons, and therefore the most recently evolved group of plants are the grasses, which belong to the family Poaceae.

The grasses are one of the most important groups of plants for mankind. From this group we use wheat (Triticum æstivum) to make flour. According to the Australian Wheat Board, Australia produced 20.5 million tonnes of wheat in 2001-2 of a total of 567.3 million tonnes grown worldwide. Other significant grasses of major economic value include corn (Zea mays), barley (Hordeum spp.), millet (Panicum sp.), rice (Oryza sativa) and oats (Avena sativa). The development of grasses was critical to the future success of man (Homo sapiens).

The first grass fossils appear in the record of the Palaeocene and early Eocene (65-50 Ma) however their widespread global expansion progressed slowly until the middle Miocene (c.20-10 Ma) where grasslands and grass-dominated ecosystems became commonplace.

Compared to other angiosperms, the evolution of grasses was slow. While there are a number of theories on why this might be, the most popular one relates to the climate of the Tertiary period.
Lolium perenne
Perennial ryegrass (Lolium perenne) growing in a glasshouse. This is a major pasture crop.
During this time, there was an increase in the aridity of many parts of the earth, especially the continental interiors and a corresponding decrease in temperatures. Since many modern grasses are well adapted to drier ecosystems, this theory is probably the most credible. Grasslands tend to occur in regions of lesser rainfall. In addition, grasses are generally well adapted to survive grazing and fire, and an increase of these two phenomena may also have helped in the selection of grasses to dominate these ecosystems.

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