The kingdom Animalia comprises a vast variety of living and extinct species.
Molecular phylogenetic analyses indicate that the common ancestor of animals evolved approximately 650 to 800 million years ago (mya).
This ancestral protist likely resembled modern choanoflagellates, which are flagellated eukaryotes.
Choanoflagellates are part of the opisthokont group, which is the sister group to clade Metazoa (animals).
Choanoflagellate Protists
Extant choanoflagellates are small, unicellular, heterotrophic protists.
There are over 125 species of unicellular or colonial forms found in marine and freshwater environments, including stalked and ball-like colonies.
They are characterized as collared flagellates (the term 'choano' means collar) possessing a funnel-shaped microvilli collar at the base of the flagellum.
Microvilli are finger-like projections of the cell membrane that help capture bacteria.
It is hypothesized that animals evolved from choanoflagellate-like ancestors.
Evidence of Relation to Animals
Three lines of evidence indicate that choanoflagellates are closely related to animals:
Cell Morphology: Choanoflagellate cells are almost indistinguishable from collar cells of sponges.
Unique Cell Morphology: Cells resembling choanoflagellates are found in animals but never in non-choanoflagellate protists, plants, or fungi.
DNA Sequence Homology: Molecular phylogenies confirm choanoflagellates as the closest extant relatives of animals.
Origins of Multicellularity
The evolution of eukaryotic cells facilitated the emergence of various unicellular forms, including protists.
Multicellularity appeared independently in multiple lineages, leading to the development of algae, plants, fungi, and animals.
The oldest multicellular eukaryote fossils (algae) date back to approximately 1.2 billion years ago (bya).
Advantages of multicellularity include:
Cell specialization
Increased size and complexity
Longer lifespans
Enhanced defense against predation
Molecular clock calculations suggest that animals originated around 650–800 mya.
Whole-body animal fossils are from the Ediacaran Period (Neoproterozoic Era).
Neoproterozoic Era (1 billion – 541 mya)
Earliest complex multicellular animal fossils are from the Ediacaran biota (635–540 mya).
The rise of marine planktonic algae (Archaeplastida) around 800–650 mya increased atmospheric oxygen levels, known as the Neoproterozoic Oxygenation Event.
This transition to algal-dominated ecosystems created wider food webs with more efficient nutrient and energy transfers, supporting the evolution of larger, complex organisms.
The Ediacaran biota consisted of diverse soft-bodied, mostly sessile marine organisms.
This biota was largely extinct by the Cambrian period.
Paleozoic Era (541–252 mya)
The Cambrian explosion (535 to 525 mya) marked significant diversification in animal evolution.
Fossils from this period show a rapid increase in diversity of bilaterian animals with bilateral symmetry and complete digestive tracts.
Some animal phyla, such as sponges, cnidarians, and mollusks, predated the Cambrian explosion.
Hypotheses for the Cambrian Explosion
Three main hypotheses explain the Cambrian explosion:
Evolution of Predation: The emergence of predator-prey relationships drove the evolution of complex body structures and defense mechanisms.
Neoproterozoic Oxygenation Event: Increased oceanic oxygen levels supported greater body sizes and energetic lifestyles, such as muscular carnivores.
Evolution of Hox Genes: The appearance of Hox genes, responsible for regulating body plans, facilitated the diversification of body forms during this period.
Animal Evolution Timeline
During the Paleozoic Era (541–252 mya):
Animal diversity increased but faced extinction events during the Ordovician, Late Devonian, and Permian periods.
Invertebrates began colonizing land around 450 mya, followed by vertebrates around 365 mya.
Mesozoic Era (252–66 mya):
Dinosaurs emerged as dominant terrestrial vertebrates; the first mammals also appeared.
Cenozoic Era (66 mya to present):
Following the Cretaceous mass extinction, mammals diversified in size and species, exploring available ecological niches.
Invertebrates
Invertebrates constitute 95% of known animal species and inhabit nearly all habitats on Earth.
A diverse range of species exists, from microscopic to large, such as the colossal squid (Mollusca), which can weigh up to 495 kg and measure 10 meters in length.
The term “invertebrates” refers to animals lacking a backbone.
Anatomical classification reveals that invertebrates are paraphyletic, meaning they do not form a clade.