The link between Fishes, Amphibians, and Ancient Plants is essential to the ‘study’ of Evolution of Life. Around 3.5 billion years ago, just like other planets, Earth was unable to support life on it because of high temperature, no free oxygen, and no atmosphere. Later on, life emerged as single-celled organisms as a result of ecological succession undergone by our Earth.
Prokaryotes were the first organisms to appear and were the sole life form for around 2000 million years since the inception of life on Earth. Researchers of Australia found the 3.48 million-year-old fossil of microorganism. Though prokaryotes are invisible to the naked human eye, their contribution to the ecosystem is commendable, which paved the way for eukaryotic life.
The First Eukaryote that Emerged is the Plant Kingdom
A multicellular alga, Grypania spiralis is the oldest eukaryotic fossil known. It was found from 2100 million years old rock in China. These photosynthetic eukaryotes were present in the sea for 1500 million years, and they developed with a patience that would allow them to stay out of water.
The fossilized spores from the Ordovician period showed the earliest evidence for the appearance of land plants. Land plants probably evolved from green algae living in freshwater ponds.
Plants depend on water for reproduction, and hence only wetter areas on land were covered with vegetation. These were the Bryophytic plants, which are known as ‘amphibians of the plant kingdom.’
This situation, along with physical conditions on Earth, forced for an evolutionary drive. Physical conditions like drought, UV, and internal factors like support and reproduction stood firm for evolution.
Successful plants then evolved their vascular systems and roots, which later developed into Pteridophytes, Gymnosperms, and Angiosperms. They dominated the terrestrial environment ever since. Life finally was able to shift to land. Plants were able to establish a niche on Earth in the last 400 million years.
The Connecting Link between Plants and Animals
Bacteria and eukaryotes (plants and animals) share a common ancestor, the Last Universal Common Ancestor (LUCA). At a cellular level, plants and animals share a common feature of having mitochondria, biochemical pathways like electron transport chain, and also in terms of bioenergetics.
Eukaryotic microalgae, Euglena gracilis, had few characteristics of plants and animals, which made the scientists confused to place the organism in the taxonomical hierarchy. Euglena is then placed in the kingdom Protista. Euglena does photosynthesis like plants and does not possess a cell wall around the cell membrane like animals.
Evolution of Vertebrates from Invertebrates
Phylogenetics provides a bit of more accurate information regarding the origin of vertebrates from invertebrates. It helps us to achieve an appropriate outcome concerning the order of particular anatomical traits acquired in the vertebrate ancestors.
Though invertebrates have a smaller size and simpler structure, compared to vertebrates, which may vary from small to the massive one, they mark their presence up to 98% of the whole animal kingdom while vertebrates occupy only 2%. Vertebrates have an advanced structural organization of the body.
All invertebrates and vertebrates belong to the phylum Non-Chordata and Chordata, respectively. The primary difference is the presence or absence of notochord between chordates and non-chordates. Non-chordates lack backbone or notochord, while chordates have a cord or spine in its structure.
Establishing a phylogenetic framework of links among organisms is essential to uncover the patterns and mechanisms of evolution. A comparison of morphological and anatomical structures provided the evidence to support evolution. Neural crest cells and neurogenic placodes are unique to vertebrates.
The peripheral nervous systems, which include sensory organs like the eyes, nasal organs, inner ear, and the lateral line system of fishes, are originated from neurogenic placodes.
Migration is one of the properties of neural crest cells. It also behaves like stem cells in that they have the potential to differentiate into a broad spectrum of specialized cell types; pharyngeal bone, cartilage, and dentine. Thus the origin of neural crest cells and neurogenic placodes led to the evolutionary emergence of vertebrates.
The Age of Fishes
Fishes first appeared in the Cambrian period, 520 million years ago. The first fish genealogies belong to the Agnatha or jawless fish, which includes Conodonts, Ostracoderms, and Lampreys. In the present scenario, excluding Lampreys, most jawless fishes are extinct. Jawless fishes probably originated from the subphylum Cephalochordata.
A notochord might harden this ancestral body. Agnathostomes cannot use new food resources as they did not possess a jaw. Jaws are an excellent example of an evolutionary advantage, which is a structure attached to the skull through temporomandibular joints.
Vertebrates with jaw are called Gnathostomes. During the late Ordovician period, the first jawed vertebrates appeared.
Placoderms, evolved from Ostracoderms and Acanthodii are the first representatives of this group during the Silurian period. Chondrichthyes (or cartilaginous fish) and Osteichthyes (or bony fish) that appeared during the Silurian period still exist today are jawed fishes.
Sharks, rays, sawfishes are contained in the clade of Chondrichthyes. They looked nearly 370 million years ago in the early or middle Devonian. There was a significant increase in the fish varieties during the period of Devonian, which made it known as the age of fishes.
Today, roughly 30,000 species of bony fishes exist, which makes this the largest class of vertebrates.
Actinopterygii (ray-finned fish) and Sarcopterygii (lobe-finned fish) are the two present clades of Osteichthyes. It was from the lobe-finned fishes; modern tetrapods evolved around 390 million years ago. Tetrapods are animals with four limbs, which include amphibians, reptiles, birds, and mammals belonging to the superclass Tetrapoda.
The Rise of Tetrapods: Transition from Fin to Limb
The Placoderms dominated the seas during the early or middle Devonian period. Also, fishes were diverse and mostly predatory. It was a fish-eat-fish world that made Sarcopterygians escape to freshwaters. One of the lineages of Sarcopterygian, the Rhipidistians, diverged into two groups, such as Lungfish and Tetrapodomorphs. These had lungs for breathing inside water.
So a question arises here as what might probably happen that led these fish with lobe fins to evolve to what we call tetrapods? During the Devonian period, the streams became shallower and strangled by plants. The flow also had low oxygen content due to decaying plants—the fish which was able to overcome this situation evolved to walk on this new Earth.
The change from fin to limb appears to have progressed as some factors were supporting this transition. Limbs relinquish the body from the sea bed to the surface of the water to get oxygen. It also helps to push the body along the bottom to move into shallower water to hunt down its prey or to escape from the bigger predators.
Lungfishes evolved the first proto-lungs and proto-limbs, which helped them to adapt to the condition outside water.
There are various hypotheses on how Lungfishes changed their proto-limbs. It was essential to shift from marine to freshwater bodies, or it might have been developed to navigate through shallow water vegetation.
The Early Amphibian
In 2004, researchers published the discovery of a fossil of Tiktaalik roseae, found on Ellesmere Island in Arctic Canada. This tetrapod-like fish is likely the mid-point between fishes having fins and tetrapod having limbs. They are thought to be lived about 375 million years ago.
The close relative of Lungfish, Tetrapodomorph, had their first member, Rhizodonts, with tetrapods or four limbs. But they were considered aquatic till late Devonian. They also evolved digits that help them to crawl along the sea bottom.
The first primitive amphibian, Ichthyostega, had two pairs of stout limbs, a skull similar to their ancestor, Eusthenopteran. They had improved lungs and nostrils for breathing, which enable them to stay on land for an extended interval.
The bones also become more substantial and stronger to support the weight of their bodies on land. Ichthyostegalians also possess sensory organ to detect vibrations and movement which present-day amphibians have. And just like amphibians today, they have the feature of metamorphosis.
Though they have well-developed lungs and strong bodies, they are dependent on water for laying eggs. Their eggs are shell-less and covered with a jelly-like substance that protects them from predators or drying out. Modern-day amphibians are descended from the Ichthyostegalian ancestors.
The Early Reptiles
During the Carboniferous period, approximately 345 million years ago, bryophytes and pteridophytes dominated the shallow water, turning them to marshy lands due to climate change. This, in turn, favored the emergence of land arthropods as the food of amphibians. Amphibians of this time are comparatively large animals with strong limbs and stout. Most animals of this period survived into the Permian period.
Reptiles, birds, and mammals are amniotes, which is a clade of tetrapod vertebrates. An egg with an amnion characterizes the name amniote. Reptiliomorphs were the ancestors of amniotes that contain small lizards that appeared about 340 million years ago.
The early amniotes diverged into two groups. The one called Synapsids evolved into mammals, and the other called Sauropsids, which evolved into reptiles and birds. These two groups differ mainly in their skull structures. Canaria is the oldest known reptile, while the earliest reptile is Hylonomus.
The Permian period also witnessed the divergence of reptiles into Archosaurs, which are predecessors of dinosaurs and crocodiles, and Lepidosaurs, predecessors of snakes and lizards.
These groups remained unnoticed until they became dominant terrestrial groups due to Permian- Triassic extinction. Archosaurs radiated into dinosaurs and had more than 1000 species classified. They were either carnivores or herbivores.
Evidence shows that they exhibit parental care, similar to Crocodilians and Archosaur birds. The age of reptiles was in the Mesozoic Era, where dinosaurs dominated 250 million years ago. Their extinction happened at the end of the Mesozoic era, the Cretaceous period. However, many reptiles survived and gave rise to modern reptiles.
Evolution chooses the ones that survive best in these new environments.
“Survival of the fittest” – Charles Darwin
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Featured Image: Link between Fishes and Amphibians and Ancient Plants
Kerala, India
M.Sc Microbiology
