Sponges and comb jellies represent the deepest-branching animal lineages on Earth, with origins stretching back at least 600 million years and possibly much further. But if the question is which recognizable animals alive today most closely resemble their ancient ancestors, the list expands to include horseshoe crabs, nautiluses, brachiopods, hagfish, and a handful of other groups whose basic body plans have remained strikingly stable across hundreds of millions of years. The answer depends on what you mean by “been around,” and the distinction turns out to be more interesting than it sounds.
The Deepest Split in the Animal Tree
Every animal alive today descends from a common ancestor that lived sometime before the Cambrian period, roughly 540 million years ago. The first branch off that trunk is either sponges or comb jellies, and biologists have been arguing about which one it is for over a decade. The traditional view holds that sponges split off first, which makes intuitive sense: sponges lack true tissues and organs, so placing them at the base of the tree supports the idea that animal complexity built up gradually over time.
Some genomic studies have challenged that picture by placing comb jellies at the base instead. If comb jellies branched off first, it would mean that either the common ancestor of all animals was more complex than sponges (and sponges later simplified), or that nervous systems and muscles evolved independently more than once. A study in the Proceedings of the National Academy of Sciences found no support for the comb-jelly-first hypothesis and concluded it was likely an artifact of overly simple analytical methods and poor choices about which species to use as reference points for rooting the tree.1PubMed Central. Genomic data do not support comb jellies as the sister group to all other animals But the debate continues. Cambrian fossils of stem-group ctenophores have confirmed that the lineage is genuinely ancient, even if its exact placement remains contested.2Current Biology. Cambrian Stem-Group Ctenophores Yield Insights into Early Ctenophore Evolution
Either way, sponges and comb jellies both trace their lineage to before the Cambrian explosion. They are not just old species; they represent entire body plans that have persisted since before most animal phyla existed. Jellyfish are close behind. Fossil polyps (the sessile stage of the jellyfish life cycle) appear in the late Ediacaran, and a recently described fossil from southern China represents the oldest known macroscopic jellyfish, dating to the early Cambrian and showing affinity with modern hydrozoans.3Communications Biology. A macroscopic jellyfish from the early Cambrian of south China
Ancient Invertebrates You Can Still Find Today
Some of the most famous “ancient animals” are invertebrates whose external appearance has barely changed over geological timescales, even as their genetics and internal biology have continued to evolve. These are the creatures that tend to land on lists of living fossils, though that label comes with caveats worth exploring.
Horseshoe crabs are among the most striking examples. The oldest known horseshoe crab fossil, a species called Lunataspis aurora, comes from Late Ordovician deposits in Manitoba, Canada, making it roughly 450 million years old. What surprised researchers is that this ancient animal was already strikingly similar to modern horseshoe crabs, meaning the basic horseshoe crab body plan locked into place far earlier in the Paleozoic than anyone had expected.4Palaeontology. The Oldest Horseshoe Crab: A New Xiphosurid from Late Ordovician Konservat‐Lagerstätten Deposits, Manitoba, Canada Today’s four surviving species belong to a lineage that has outlasted multiple mass extinctions.
Nautiluses are another iconic survivor. Their chambered shells are nearly identical to fossils hundreds of millions of years old, but the group’s real claim to fame is surviving the asteroid impact that killed the dinosaurs 66 million years ago. Research on the metabolic rates of nautiloids compared to their relatives, the ammonoids (which went extinct at the same event), found that the nautiloid Eutrephoceras had a significantly lower metabolic rate than co-occurring ammonoid species. That frugal metabolism likely helped nautiloids ride out the collapse of ocean plankton populations that followed the Chicxulub impact.5Geology. Ammonoid extinction versus nautiloid survival: Is metabolism responsible?
Tadpole shrimps (Triops) look almost identical to fossils more than 170 million years old, which is why they are a textbook example of morphological stasis.6Biological Journal of the Linnean Society. Morphological stasis and phylogenetic relationships in Tadpole shrimps, Triops (Crustacea: Notostraca) But molecular work has complicated the story. A global phylogenetic analysis found no molecular evidence for an ancient pre-Mesozoic radiation of living tadpole shrimps. Instead, the species alive today appear to be the product of a much more recent radiation during the Cenozoic era; they just happen to look almost exactly like their distant ancestors because the body plan is so conserved.7PLOS ONE. Toward a Global Phylogeny of the “Living Fossil” Crustacean Order of the Notostraca This is a recurring theme: looking old and being old are not the same thing.
Brachiopods of the genus Lingula round out the invertebrate list. These small, shelled marine animals have a fossil record stretching back to the early Cambrian, and modern Lingula shells look remarkably like specimens from over 500 million years ago. Genomic analysis has revealed, however, that their genome is anything but static. Lingula has expanded its gene count to roughly 34,000 through extensive duplication of gene families, suggesting that the outward appearance of changelessness masks a busy evolutionary history at the molecular level.8PubMed Central. The Lingula genome provides insights into brachiopod evolution and the origin of phosphate biomineralization
Velvet worms (onychophorans) also belong on this list. Their Cambrian relatives, the lobopodians, were a diverse group. One fossil from China, Collinsium ciliosum, was a heavily armored lobopodian within the stem group of modern velvet worms, and the range of body forms among these ancient relatives was substantially greater than what exists among velvet worms today.9PubMed Central. A superarmored lobopodian from the Cambrian of China and early disparity in the evolution of Onychophora Modern velvet worms are the conservative survivors of a once-varied group.
Ancient Vertebrates Still Swimming and Crawling
Among vertebrates, hagfish and lampreys occupy the oldest lineages. These jawless fish split from the rest of the vertebrate tree before jaws, paired fins, or bony skeletons evolved. A Cretaceous-age hagfish fossil from what was once the Tethys Sea confirmed that the distinctive features of modern hagfish, including their slime glands and degenerate eyes, were already in place well before the Cretaceous. More importantly, the fossil helped resolve a long-running conflict between molecular and morphological data by supporting the hypothesis that hagfish and lampreys form a single group (cyclostomes), rather than hagfish being a more distant branch.10PubMed Central. Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological-molecular conflict in early vertebrate phylogeny
Lampreys have a total-group history reaching back to the Ordovician, over 440 million years ago. But the living species are younger than they appear. A phylogenomic study estimated that crown-group lampreys (the group containing all living species) originated in the Cretaceous, with a mean age of about 94 million years. Northern Hemisphere lampreys are younger still, originating around the Oligocene-Miocene boundary roughly 24 million years ago, with some genera emerging as recently as the Pleistocene.11PubMed Central. Phylogenomic resolution of lampreys reveals the recent evolution of an ancient vertebrate lineage So the lamprey body plan is ancient, but the species wearing it are comparatively new.
Coelacanths are probably the most famous “living fossil” among vertebrates. Thought to have gone extinct with the dinosaurs until a living specimen was caught off South Africa in 1938, coelacanths have become a symbol of evolutionary persistence. Genome sequencing confirmed that their DNA evolves unusually slowly, with extremely low genetic diversity among living individuals suggesting either a small population or genuinely sluggish molecular evolution.12PubMed Central. Coelacanth genomes reveal signatures for evolutionary transition from water to land Their HOX gene clusters, which control body patterning during development, evolve more slowly than those of other vertebrates, reinforcing the picture of a lineage in which change happens at a crawl.13PubMed Central. Complete HOX cluster characterization of the coelacanth provides further evidence for slow evolution of its genome
Deep-sea sharks of the order Hexanchiformes, which includes the frilled shark and the sixgill and sevengill sharks, also represent an ancient vertebrate lineage. Phylogenetic analysis of their mitochondrial genomes placed them within the superorder Squalomorphi and resolved the frilled shark as the sister species to all other members of the group.14Hindawi / Biomedicine Research International (PubMed Central). Evolutionary relations of Hexanchiformes deep-sea sharks elucidated by whole mitochondrial genome sequences These sharks retain features, like extra gill slits and a single dorsal fin, that are considered ancestral among sharks, and their fossil record extends deep into the Mesozoic.
Why “Living Fossil” Is a Loaded Term
The phrase “living fossil,” coined by Darwin, implies that a species has stopped evolving. In practice, no species stops evolving. What changes is how visible that evolution is. The tuatara of New Zealand is a case study. It looks superficially like a lizard and is the sole surviving member of the order Rhynchocephalia, which was diverse during the Mesozoic. Researchers have confirmed that the tuatara sits near the center of the morphological range for all rhynchocephalians and has undergone unusually slow lineage evolution.15Palaeontology. Macroevolutionary patterns in Rhynchocephalia: is the tuatara (Sphenodon punctatus) a living fossil? But a separate analysis demonstrated dramatic differences between the tuatara’s skull and that of its Cretaceous relative Priosphenodon, and the Mesozoic members of the group displayed a much wider range of body forms than the tuatara represents.16PubMed. Tooth and cranial disparity in the fossil relatives of Sphenodon (Rhynchocephalia) dispute the persistent ‘living fossil’ label The tuatara is not frozen in time. It is the last surviving member of a group that was once diverse, and its apparent conservatism masks the extinction of all the relatives that were different.
The same pattern applies to tadpole shrimps, brachiopods, and coelacanths. Outward resemblance to fossils does not mean genetic or ecological stasis. In many cases, the species that survived are the generalists, the ones occupying a broad ecological niche that buffered them against environmental change. Broad resource use and niche breadth act as insurance against extinction, allowing generalist lineages to persist even as specialist relatives disappear.17Oxford Academic. The generalism–specialism debate: the role of generalists in the life and death of species
When the Fossil Record and Molecular Clocks Disagree
One complication in identifying the oldest animal lineages is that the fossil record and molecular-clock estimates often tell different stories. The oldest unambiguous fossils of bilaterally symmetrical animals date to about 555 million years ago, but molecular clocks calibrated using vertebrate fossils have placed the split between vertebrates and insects at roughly 900 million years ago.18PubMed Central. Estimating metazoan divergence times with a molecular clock That is a gap of more than 300 million years.
The disagreement has several sources. Molecular estimates are sensitive to the models used and to variation in how quickly different lineages accumulate mutations. A review in BioEssays argued that the gap is not as large as it first appears: modern clock analyses do not actually predict that fully formed representatives of most animal phyla were walking around in the Neoproterozoic. What they predict is that the lineages leading to those phyla had begun to diverge, which is compatible with a sparse early fossil record if the earliest animals were small, soft-bodied, and unlikely to fossilize.19PubMed. The origin of animals: Can molecular clocks and the fossil record be reconciled? Other researchers have shown that limited gene sampling and uneven evolutionary rates among species are the most likely sources of the discrepancy.20PubMed Central. The timing of eukaryotic evolution: does a relaxed molecular clock reconcile proteins and fossils?
Exceptional fossil sites, known as Lagerstätten, have helped narrow the gap by preserving soft-bodied organisms that normally leave no trace. These deposits retain a disproportionately large amount of biological information compared to typical fossil beds.21Paleobiology. Fossil Lagerstätten and the enigma of anactualistic fossil preservation Without them, our picture of early animal life would be even more incomplete, and many of the “oldest” animals discussed here would have no fossil record at all.
Lineage Longevity Versus Individual Immortality
The question of which animals have been around the longest usually refers to lineages, but there is a parallel question about individual lifespans that often gets tangled up with it. The ocean quahog clam (Arctica islandica) holds the record for the longest-lived individual non-colonial animal, with some specimens exceeding 500 years. Research on populations from different regions found that extreme longevity in this species appears linked to very low metabolic rates combined with stable antioxidant defenses throughout adult life, rather than to any especially powerful antioxidant system on its own.22Aquatic Biology. Age-dependent patterns of antioxidants in Arctica islandica from six regionally separate populations with different lifespans
Then there is Turritopsis dohrnii, the so-called immortal jellyfish, which can reverse its life cycle from the adult medusa stage back to the juvenile polyp stage through a process called transdifferentiation.23PubMed. Regenerative characteristics of the immortal jellyfish, Turritopsis dohrnii, and their potential implications for human aging Genomic studies have identified specific genes involved in this reversal, including genes related to DNA repair, lifespan regulation, and the silencing of certain developmental controls, which together allow the jellyfish to essentially reset its cells to a pluripotent state.24PubMed Central. Comparative genomics of mortal and immortal cnidarians unveils novel keys behind rejuvenation This is biologically fascinating but has little to do with lineage antiquity. Turritopsis is a relatively ordinary hydrozoan in evolutionary terms; its trick is individual survival, not species persistence across geological time.
Life in Extreme Environments
Some ancient lineages have persisted partly because they colonized environments that buffer them from the competitive pressures and ecological upheavals happening closer to the surface. Deep-sea hydrothermal vents host polychaete worms of the family Alvinellidae, and genomic work on their protein evolution revealed something unexpected: the ancestor of all living alvinellid species was likely thermophilic (heat-loving), and lineages that still live near high-temperature vents have maintained that adaptation through strong purifying selection. The species that later moved to cooler habitats show signs of selective relaxation and some positive selection for cold tolerance at the protein level.25PubMed Central. Proteome Evolution of Deep-Sea Hydrothermal Vent Alvinellid Polychaetes Supports the Ancestry of Thermophily and Subsequent Adaptation to Cold in Some Lineages In other words, the deep-sea extreme was the starting condition, not a later refuge. These animals did not flee to the vents to escape something; they started there and some eventually left.
This matters for understanding which animals persist longest, because it flips the usual narrative. We tend to assume that extreme environments are marginal habitats where oddball species eke out a living. For some lineages, the extreme environment was home base, and the stability of that environment (hydrothermal vents have existed for billions of years, even if individual vents are short-lived) may have contributed to the lineage’s persistence. The deep ocean changes slowly compared to shallow seas and land surfaces, and animals adapted to its conditions face fewer of the abrupt climate swings that drive mass extinctions in other habitats.