What is Marine Evolution?

Marine habitat is less explored, and hence, its biological diversity is far less known. The marine habitats are not constant due to their shifting nature, while the terrestrial habitats are more stable. Marine biology is the science that deals with the study of diverse marine species, their interactions, and behavioral characteristics.

All life originated from microscopic forms and evolved into macroscopic and humongous creatures. The life beneath the waves is subjected to changes leading to marine evolution. Marine evolutionary biology is a science that deals with the study of taxonomic classification, morphology, phylogenetic aspects, and evolution of life undersea and its biodiversity.

What is the classification of marine life forms?

Marine life has three domains of classification: bacteria, archaea, and eukarya. Bacteria and archaea are prokaryotic organisms. They include cyanobacteria and Methanococcus. Eukarya are eukaryotic organisms. They include protists, fungi, plants, and animals. Examples include marine algae or seaweeds, mushrooms, marine plants like seagrass, and animals, including invertebrates and vertebrates.

An illustration represents the three domains of life. A phylogenetic tree with branches show three domains of life that are Bacteria like Cyanobacteria, Archaea like Methanococcus, and Eukarya like Fungi, Plantae, and Animalia.
Three domains of life

Marine evolution through geological time scale

Many marine species existed only for a limited geological time. In contrast, some other species evolved with the changing seas and exist now. Mass extinction occurs when species fail to evolve in response to rapidly changing marine habitats. Notable evolution in some eras are described below:

A step-diagram shows geological eras. The image displays a staircase pattern showing the oldest Precambrian era at the base. It is subsequently followed by Paleozoic, Mesozoic, and Cenozoic eras on the geological time scale.
Geological era

Precambrian era

Microbes were the first lifeforms that originated in the marine habitat about 3.5 billion years ago. They lived near hot spaces and hydrothermal vents. Mineral-rich water formed the main basis of nutrition and energy for such lifeforms. In this era, there was no oxygen.

One of the gases available was hydrogen sulfide. These microbes use sulfur to produce energy. In the present day, microbes still exist near hydrothermal vents. At this point, the sunlight cannot penetrate. As a result, the organisms carry out similar chemical reactions to obtain energy.

The photosynthetic bacteria emerged 2.3 billion years ago. These were cyanobacteria (also called blue-green algae) that used sunlight to perform photosynthesis and obtain energy. They also released oxygen in the process.

Around 560 million years ago, complex marine organisms existed that were different from the present-day lifeforms. They had a soft tube-like body with an outer sheath. The seafloor was covered by marine organisms that resembled ferns. These fern-like fronds absorbed simple molecules and produced energy by using simple chemical substances. Burrowers originated in this era. They feed on microbes and are believed to be the ancestors of present-day species.

Paleozoic era

It is the era when marine habitats flooded the continents. Shallow water marine habitats originated and supported marine life. The species were larger and more complex than their ancestors. Sponge-like creatures arose and became the reef builders of the sea. Cambrian-age fossils prove the emergence of such species.

Organisms with shells also developed. Shells helped in protection and defense against predators. Shelled animals are related to the present-day mollusks and worms. The most notable species were trilobites. They were the close relatives of insects, crabs, and spiders. These invertebrate creatures with segmented bodies dominated the seabed.

Jawless fish emerged as the first vertebrate. This era also witnessed the emergence of the first marine predators with crushing jaws. Similar to the present-day whale shark, there were shrimp-like organisms that were filter-feeders. These organisms had specialized structures to trap planktons.

Mesozoic era

This era witnessed the domination of reptiles in the sea. They fed on bivalves, fishes, and cephalopods. Marine turtles resembling the present-day sea turtles emerged. Present-day planktons also emerged in this era. Structures like spines, spikes, and shells in organisms evolved into modern-day teeth and claws.

Cenozoic era

It is the age of mammals dominating all the habitats. In the marine habitat, marine mammals dominated. Baleen whales emerged in large proportion. Four-legged mammals similar to seals and sea lions of the modern-day lived among seagrass and kelps. However, these species with flippers gradually became extinct. Aquatic orders of marine mammals such as cetaceans, pinnipeds, and sirenians underwent convergent evolution and developed similar body structures. Whales and dolphins belong to cetaceans, sea lions and walruses belong to pinnipeds, and dugongs belong to the order sirenians.

Marine evolution in present day

The present-day is the Anthropocene epoch that includes the records of the impacts of human activities on the ecosystem. At present, the lifeforms in seas and oceans are adapting to the changing habitat as human activities constantly manipulate sea and oceanic ecosystems. Activities such as the development of coastlines, overfishing, and introductions of invasive species interfere with the evolutionary processes and decrease species diversity.

Human activities strongly influence climate too. Pollution caused by car emissions and power plant installation is responsible for climate change. As the atmosphere is warming up due to climate change, life in the seas and oceans is threatened. The dumping of non-biodegradable materials like plastics and oil spills leads to the death of several aquatic lifeforms. The concentration of carbon dioxide has also increased, leading to the acidification of seas and oceans. Biodiversity is thus deteriorating at an alarming rate.

An illustration represents the effects of anthropogenic factors on marine life. Global warming, car emissions, plastics dumping, and climate change are shown in four boxes drawn on a circle. These phenomena lead to reduced population of marine organisms, death of marine organisms, and reduced biodiversity shown inside an inner circle.
Anthropogenic impacts on marine life and evolution

Biodiversity and extinction

The present-day human activities are changing the marine environment and aquatic life faster than during geologic time. It is high time to take actions and measures to rectify the impacts of past activities and use sustainable measures in the present. Conservation of marine life is necessary, and failure to do such would lead to mass extinctions. As a result, the trophic structure and marine ecosystem would be affected badly. Both species diversity and biodiversity would fall, thus disrupting the ecological balance.

Common mistakes

Students may make the following errors:

  • Marine habitat includes seas and oceans and the coastal areas from the edge of the continental shelf. Marine habitat also includes seabeds, oceanic floors, and the areas till the waves reach.
  • Marine organisms are also found in estuaries, where freshwater and saline water meet.
  • The majority of fishes are found in the marine ecosystem and not in freshwater.
  • Marine animals are found in tropical, temperate, polar, and deep-sea regions.
  • Oceans may not appear to be changing, but they undergo several modifications that can lead to both evolution and extinction.

Context and Applications

The topic is significant for both undergraduate, graduate, and doctoral courses, especially:

  • Bachelor of Science in Marine Biology
  • Bachelor of Science and Master of Science in Oceanography
  • Bachelor of Science and Master of Science in Aquatic Ecology
  • Doctor of Philosophy in Oceanography
  • Evolutionary Studies
  • Marine ecosystem: ecology and evolution
  • Evolution of freshwater ecosystem
  • Evolution of whales
  • Marine biodiversity

Practice Problems

Q1: How many years ago did life originate on Earth?

(a) 3.5 million years ago

(b) 3.5 billion years ago

(c) 2.3 billion years ago

(d) 2.3 million years ago

Correct choice: (b)

Q2: The marine habitat includes _________.

(a) sea and oceans

(b) coastal areas

(c) estuaries

(d) all of the above

Correct choice: (d)

Q3: The geological time scale includes all eras except the ________ period.

(a) Paleozoic

(b) Cenozoic

(c) Cambrian

(d) Precambrian

Correct choice: (c)

Q4: Which class of organisms dominated marine evolution in the Cenozoic era?

(a) Mammals

(b) Reptiles

(c) Amphibia

(d) Fish

Correct choice: (a)

Q5: In the present day, _______ influences marine evolution.

(a) anthropogenic factors

(b) global warming

(c) acidification

(d) all of the above

Correct choice: (d)

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