paleontology 3: land ho!! (v1.0)

 Up to now all the transitions we talked about are sea based. In this essay we explore the transition to land. These involve algae, fungi, plants and animals and are driven by ecological factors. This is a transition that occurred over tens of millions of years. There are two major evolutionary transitions included here because all terrestrial ecosystems and everything in them we have today reflects these transitions.

In the Cambrian there was only 15% oxygen available in the air, compared to 20% today. Land animals breathe oxygen. In that time, there was a super continent Gondwana in the southern hemisphere, with a few land masses close by. The north was mostly water. The land is completely barren, with no shade and very hot. The air is empty of life. Puddles or small water bodies may have some simple algae. The only other life is some single celled Prokaryotes (example bacteria, cyanobacteria). 

The challenges for life emerging from the sea to land include prevent dehydration, tolerate temperature extremes, “stand up” under greater gravitational challenges (since no water buoyancy), reproduce somehow, take in oxygen without dehydrating (gills for example are terrible outside water), and life must find suitable food. The available food possibly for animals are Prokaryotes, algae, fungi, and lichen (which are symbiotic colonies of algae or cyanobacteria and fungi), and later in time simple arthropods – an animal: oldest arthropod land fossil is 428 million years ago.

The transition to vascular land plants is a major evolution. How did land plants evolve? Besides Prokaryotes, likely algae, fungi (not a plant), and lichens came early, and non-vascular plants like mosses, hornworts and liverworts came soon after and reproduced by spores. They release chemicals which break the rock into grains. CO2 dissolved in rainwater creates a weak acid which reacts with the rock minerals, washing some away and turning the residue into clay. Soil bacteria fix nitrogen into compounds which are useful to plants.  These laid the foundation for more complex vascular plants. The oldest known vascular plant fossil on land is 420 million years ago. Vascular plants went on to achieve great success on land because vascular tissue offers major advantages on land.

What about animals on land? Centipedes diverged from millipedes about 440 million years ago. They are arthropods. These, along with other arthropods, were early animals on land. Evidence indicates such animals may have been crawling around on land before the establishment of vascular plants. It is possible they were at the boundary between land and sea moving back and forth.

Compared to the Cambrian period, the Silurian period (from 443 million years to 419 million years ago) would have seen less soil erosion, more nutrient cycling but not very deep, fewer fluctuations and alterations in the hydrologic cycle, and less CO2 and more oxygen in the atmosphere. So, evolution resulted in some changes to the ecosystem. Insects, which are arthropods (invertebrate animals with jointed legs), which came soon after to land, also caused significant ecosystem change.

Insects belong to Hexapoda, which is a arthropod subgroup with six legs (example spiders, scorpions, ants, roaches, ticks and mites). It has a body divided into a head, a thorax and abdomen. It has an exoskeleton of chitin, two antennas and compound eyes. They have a marvelous respiratory system called the tracheal system. The study of insects is called entomology. Insects today are the most numerous macro lifeform. There are at least 5 million living species. So, it is an extraordinary evolutionary success story.  Insects are very successful because they have high reproductive rates, huge number of offsprings and rapid response to selection pressures. Insects were the first to demonstrate powered flight. There are also wingless insects. Only a small fraction of insects is marine or intertidal. The earliest insect fossil is from 400 million years ago (Devonian period – 419 million to 358 million years ago), and there is a possibility it had flight (still controversial). The oldest definitely known flying insect fossil is from 325 million years ago which is the Mississippian subperiod of the carboniferous period (358 million to 323 million years ago). Insects and insect flight was a major evolutionary change. The closest living relative of insects based on genetic analysis is crustaceans known as remipedes.

This essay covers the move to land and two major evolutions namely rise of vascular plants and the rise of insects and powered flight. This spans over a large stretch of time stretching from the Cambrian period to the Mississippian subperiod of the carboniferous period.