The major breakthroughs of plant evolution

 As plant life evolved, several major breakthroughs allowed them to greatly expand their footprint across the globe. These breakthroughs were major macroevolutionary shifts brought about by a series of small microevolutionary adaptations. The essential characteristics of Plants are each associated with one or more of these major breakthroughs. Such events are described in more detail in Plant Life: a Brief History, I present here a brief synopsis of those major events:

The earliest known fossil cyanobacteria
formed layered colonies that slowly
built pillar-like formations called
 stromatolites, like these from
present-day Australia. Photo by Paul
Harrison, CC BY-SA 3.0

1. Origin of photosynthesis - this central plant process not only marked the beginning of plant life, but also opened up a vast new energy supply to all life on earth and providing the oxygen supply that allowed for complex food webs and distinctive ecosystems. Though seemingly a long, complex process, different parts of  photosynthesis  evolved separately in more ancient bacteria and were brought together through horizontal gene transfer.  Carbon-fixation or the Calvin Cycle, had its roots in earlier chemoautotrophic organisms, where it was driven, not by sunlight, but by energy captured from sulfur and other compounds bubbling up from undersea volcanic vents. The ability to capture sunlight evolved among other bacteria, likely producing only ATP as its product. When coupled with the carbon-fixation process ,simple forms of photosynthesis came into being. The first organisms capable of modern photosynthesis, which releases oxygen as a byproduct, were the Cyanobacteria, which are still abundant today. Solid evidence of their existence goes back nearly 3 billion years, but they may have been present even earlier. (The first plants) (Cyanobacteria - the super heroes of evolution)

Chlamydomonas, a single-
celled alga CC by-SA 2.0

3. Origin of eukaryotic algae -  Primitive animal-like cells, already equipped with mitochondria, captured cyanobacteria through endosymbiosis, which were "domesticated" to become chloroplasts. (Plants and animals and kleptoplasts - oh my!) This occurred a number of times, resulting in multiple unrelated organisms called algae,  which at first floated as part of the phytoplankton of the seas. Sexual reproduction via cells specialized as sperm and egg evolved in these early algae, along with mitosis and meiosis.

Freshwater charophytes
are related to land plants

4. Origin of multicellular plants - With cells remaining attached to one another, and usually also anchored to rocks and other substrates, multicellular algae were able to branch into extensive light-gathering antenna systems, resulting in the various kinds of seaweeds and freshwater plants like charophytes.

Mosses were among the
earliest land plants, and
continue to thrive in moist
habitats. Modern Sphagnum
mosses pictured here form
vast peat bogs, particularly 
in boreal regions.

5. Invasion of the land - Green algae adapted already to freshwater habitats, colonized the land, becoming the ancestors of both bryophytes (mosses, liverworts, and hornworts) and tracheophytes  (vascular plants like ferns, gymnosperms, angiosperms). Early land plants survived by developing water-retaining outer layers and internal systems for storing and transporting water. While such plants remained close to bodies of water at first, they created the vegetation that supported the first animal life to leave the water. The hydrostatic, or turgor, pressure within terrestrial plant cells maintains cell and tissue rigidity and drives cell expansion. It also drives the transport of food-laden fluid in the phloem tissue water and, in combination with evaporation and transpiration, helps drive the movement of water from the roots to the leafy plant tops, even in trees 100 meters tall. (How does water get to the top of a redwood tree?) Turgor pressure is also the basis of plant movements, such as the closing of of leaf traps in the Venus fly trap.  (How plants do everything without moving a muscle?)

Ferns produce wind-dispersed spores
that sprout into gametophytes.

6. Invention of wind-dispersed spores.  In the earliest land plants, and still in modern bryophytes and seedless vascular plants like ferns, sexual reproduction was essentially unchanged from what it was in aquatic algae. Sperm cells had to swim to eggs through water-filled channels and films in the soil. Since the distance sperm cells could travel was very limited, early plants produced dormant, wind-dispersed spores through meiosis from diploid sporophyte plants that developed from fertilized eggs. Spores could carry genetic information between populations, thus promoting genetic diversity and greater adaptability.  Spores germinated into haploid gametophyte plants that produced another round of sperm and egg.(The truth about sex in plants) 

Ovules contain the stages of
reproduction, from spore,
to gametophyte, to embryo
surrounded by food (seed). 
In this cycad, ovules are
borne on modified leaf-like
structures.

6. Evolution of the seed - The seed, called in its early development an ovule, is both a chamber for internal sexual reproduction and a vehicle for the dispersal of the embryo once it matures. Eggs are produced by highly reduced gametophyte plants within the ovules, while sperm cells are produced by even smaller gametophyte plants within specialized spores called pollen grains, which are brought to the ovule by wind or insects.  This liberates plants from the need for water-transmission of sperm to egg, enabling them to live and reproduce in drier habitats, the same way as internal fertilization and laying of desiccation-resistant eggs allowed reptiles to spread through dry terrestrial habitats. The earliest seed plants, took the form of seed ferns, in which pollen-producing sporangia (pollen sacs) and ovules were borne directly on large, fern-like leaves. In more advanced gymnosperms, pollen and ovule forming leaves became distinct from the vegetative leaves and took different forms, most often scale-like structures grouped into strobili (cones if rigid, catkins if soft and flexible). Among living gymnosperms, only some cycads have ovule-bearing structures that still resemble leaves. 

Magnolia flowers have numerous distinct
carpels (uppermost), numerous stamens, all
subtended by a number of tepals (petals/sepals)

7. Origin of the flower -  flowers evolved as a means of manipulating insects and other animals for transfer of pollen from one plant to another (pollination). The seemingly endless  diversity of flowers is reflected in an equal diversity of  insects, birds, and other animals adapted to recognize and feed in  flowers with specific combinations of shape, color, fragrance, and nectar production. Flowering plants, or angiosperms, evolved from ancient seed ferns in parallel with the various groups of modern gymnosperms. Their pollen-bearing structures (stamens) and ovule-bearing structures (carpels), are surrounded by leaf-like petals and sepals, and arranged in a distinctive order in each flower. Carpels mature as fruits that aid in the dispersal of seeds. (What's so primitive about Amborella?)

Grasses dominate extensive areas with
alternating wet and dry seasons. Photo by V. S.
Dustin CC BY SA-3.0

8. Re-evolution of the herbaceous habit -  While ferns and other non-seed-bearing plants were herbaceous, early seed plants and all gymnosperms are woody, as required for the slow development and maturation of their seeds. Angiosperms further shortened the reproductive cycle, so as to make quick-growing, winter-dormant herbs possible again. The most significant group of angiosperm herbs are the monocots, with grasses being the most widespread and ecologically significant herbs on the planet. (How the grass leaf got its stripes) (The grasses that would be trees) Grasses support vast food webs on seasonally dry savannas, and their seeds provide the major source of sustenance for humans around the globe. 

9. Evolution of varied secondary plant compounds - All through the evolution of plants, which are both nutritious and immobile, animals evolved to feed upon them. While some plants, like many algae and grasses, could multiply fast enough to overcome such predation, many other plants have evolved  deterrents, including hard fibers and various forms of spines, thorns, etc., but most importantly toxic or repellant chemicals. Plant chemicals that deter animal herbivores have become numerous and diverse as different species of animals developed immunity to some but not all. By nature, secondary plant compounds are physiologically active and while poisonous in some circumstances, often have valuable medicinal effects. As such, they have been vital to the survival of the human species. (Medicinal plants in our own backyard)