3. Invasion of the Land

The biggest problem that organisms faced when they first colonised the land, was the air. In the water, organisms 'breathed' by absorbing oxygen through their body surfaces or through delicate structures such as gills, which needed moisture to operate. On land, these ways of breathing would not work. Air also dried out the surfaces of organisms. The third problem they faced on land, was that air did not support their bodies against gravity as much as water did. To live successfully on land, organisms needed to combat the problems that life in the air presented.

First Land Invertebrates
The first animals to move out of the seas were probably the eurypterids, or water scorpions. These were fearsome predators that reached lengths of two metres or more, making them the largest arthropods that have ever lived. Many of them had large claws for grasping prey. The oldest known eurypterids, from the Ordovician Period (490 to 434 million years ago), were marine, but by Silurian times (434 to 410 million years ago) some had migrated into brackish or freshwater environments, and some were even amphibious as shown by their development of lungs. However, none became adapted completely to a life on dry land.

First Land Vertebrates—the Evolution of Limbs

Ray-Finned Fish. Artist: Kate Nolan. Source: Museum Victoria.

The first fossils of land dwelling (terrestrial) vertebrates are found in the Late Devonian Period (around 370 million years ago). These early land dwellers evolved from bony fishes. There are two major groups of fishes: (a) sharks and rays, which have a skeleton of cartilage; and (b) bony fishes that have a skeleton of bone. Bony fishes can be divided into two groups: (i) the ray-finned fishes whose fins are supported by delicate rods only; and (ii) the lobe-finned fishes with a few large bones as well as rods, in their fins.

Today, the ray-finned fishes plus the sharks, skates and rays dominate the sea and freshwater, but it was the lobe-finned fishes which gave rise to terrestrial vertebrates. The bones in the fins of the lobe-finned fishes were able to support the weight of the first vertebrates to venture onto land, and in time evolved into the limbs of terrestrial vertebrates.

Lobe-Finned Fish. Artist: Kate Nolan. Source: Museum Victoria.

Primitive Amphibian. Artist: Kate Nolan. Source: Museum Victoria.

Evolution of Lungs
Lungs first evolved in fishes about 400 million years ago as simple air sacs that helped to control buoyancy. In a few groups of fishes, the air sacs developed blood vessels that allowed the exchange of gas between the air and fishes' bodies. Some palaeontologists think that fishes used these primitive lungs to survive in stagnant pools and during droughts.

Air Sac. Artist: Kate Nolan. Source: Museum Victoria.

Primitive Lung. Artist: Kate Nolan. Source: Museum Victoria.

First Land Plants
The first plants that colonised the land needed support against gravity, protection from dehydration, and a way to move nutrients throughout their bodies. To carry out these functions, almost all land plants evolved rigid hollow (vascular) tubes inside. These tubes provided a rigid framework, and a pathway to move liquids and nutrients to all parts of the plant. As well, small openings in the plants' surface (called stomata) evolved which could be opened and closed to regulate the flow of gases and water. Plants with these stomata and vascular tubes are known as vascular plants, and include almost all of the present day species of land plants.

Baragwanathia is one of the earliest land plants known, and at over half a metre in length, was one of the largest plants of its time. It has been found in late Silurian period rocks, around 415 million years old, from many sites in eastern central Victoria. Baragwanathia is thought to have lived in swampy areas with its roots in the water, much like the present day lycopods (club mosses). The earliest land plants elsewhere are quite rare and very small.

Genoa Amphibian. Artist: Kate Nolan. Source: Museum Victoria.

Early Amphibians
Amphibians evolved from fish near the end of the Devonian period, about 370 million years ago. In Australia, we have evidence of these early amphibians from fossilised trackways from Genoa, in eastern Victoria, as well as a fossilised bone from New South Wales. These early amphibians, known as Labyrinthodonts, did not look like present day amphibians; they were large animals with long snouts and tails, more like an alligator in appearance. Fossil amphibians from the late Devonian have also been found in other parts of the world.

Dicynodon. Artist: Kate Nolan. Source: Museum Victoria.

Mammal-like Reptiles
During the late Palaeozoic, the dominant land vertebrates were the mammal-like reptiles. Mammal-like reptiles appeared about 295 million years ago during the Carboniferous period, and became extinct 165 million years ago in the middle of the Jurassic period. They were most common during the Permian (298 to 251 million years ago), and the early Triassic. True mammals evolved from mammal-like reptiles in the Late Triassic about 210 million years ago.