Humans and trees are genuinely, biologically related. Every oak, pine, and cherry blossom shares a common ancestor with you, one that lived roughly 1.5 billion years ago. That shared heritage left deep marks in both lineages: we use many of the same core genes, similar cellular machinery, and even some of the same signaling molecules. The relationship is far more concrete than a loose metaphor about “the web of life,” and the specifics are stranger and more interesting than most people expect.
The Ancestor You Share With an Oak Tree
All life on Earth that has complex cells, a group called eukaryotes, descends from a single ancestral population. Humans, trees, mushrooms, and amoebas all trace back to this last eukaryotic common ancestor, which lived over a billion years ago. Molecular clock analyses estimate that the three-way split among the lineages leading to plants, animals, and fungi happened around 1.58 billion years ago, give or take about 88 million years.1PubMed Central. Divergence time estimates for the early history of animal phyla and the origin of plants, animals and fungi That is an almost incomprehensible stretch of time, but the biological traces of the shared starting point remain visible in both humans and trees today.
That ancestor was not a simple blob. Reconstructions of its genome suggest it was already a fairly complex organism with a gene architecture resembling that of modern multicellular life. Its genes were structured with introns at a density within roughly 53 to 74 percent of what humans have today.2PubMed. Whence genes in pieces: reconstruction of the exon-intron gene structures of the last eukaryotic common ancestor and other ancestral eukaryotes A separate analysis estimated that the plant-animal ancestor already had intron density about two-thirds that of modern humans, and that roughly two-fifths of the introns found today in plants, animals, and fungi trace all the way back to that shared ancestor.3PubMed Central. Complex early genes In other words, the basic architecture of your genes and those of a redwood were already taking shape before the two lineages went their separate ways.
Shared Cellular Hardware
Look inside a human cell and a tree cell, and you will find the same fundamental toolkit doing the same fundamental jobs. Actin and tubulin, the proteins that form the internal scaffolding of cells, are conserved across the full sweep of eukaryotic life.4Indian Journal of Genetics and Plant Breeding. Phylogeny of actin and tubulin gene homologs in diverse eukaryotic species These are not vaguely similar proteins doing unrelated tasks. They are recognizably the same molecules performing the same structural role: holding cells together and helping them divide. When a plant cell splits in two during growth, it relies on tubulin in much the same way your cells do during wound healing or normal tissue maintenance.
Histones, the spool-like proteins that DNA wraps around for packaging and regulation, are another shared inheritance. The resemblance between plant and animal histones has been documented since the 1960s, and the similarity is not superficial. These proteins have barely changed over a billion-plus years of evolution because they perform a job so essential that almost any mutation would be harmful. Your chromosomes and those of a maple tree are organized using essentially the same packaging system.
Then there are mitochondria, the energy-producing structures inside your cells. Mitochondria arose from an ancient bacterial cell that was engulfed by the ancestor of all eukaryotes and never left. Genomic evidence confirms that this organelle originated from within a group of bacteria called Alphaproteobacteria and has been passed down to every eukaryotic lineage since.5PubMed Central. Mitochondrial evolution The mitochondria in your muscle cells and those in the root cells of a pine tree descend from the same ancient symbiotic event. Research suggests this event happened around the same time the eukaryotic cell itself was taking shape, rather than being a later addition.6PubMed. Mitochondrial evolution
Genes That Still Speak the Same Language
Because humans and trees descend from the same ancestor, we share thousands of genes. These are not just genes that happen to look alike. Many are direct descendants of the same gene in the common ancestor, doing the same job in both organisms after more than a billion years of independent evolution. Researchers recognized early on that comparing the amino acid sequences of proteins from distantly related species could reveal evolutionary relationships, because changes accumulate at roughly predictable rates over time.7Nature. Higher Plant Cytochrome c Cytochrome c, a small protein involved in energy production inside mitochondria, became one of the classic examples: the version in wheat and the version in humans are clearly recognizable as the same protein, differing at a modest fraction of their amino acid positions despite the vast evolutionary distance.
The same pattern shows up across a wide range of molecular machinery. Genes involved in DNA repair, cell division, protein folding, and basic metabolism are shared between plant and animal lineages. When a tree’s cells detect damaged DNA and activate repair enzymes, many of the genes orchestrating that repair have direct counterparts in your cells doing the same thing. This is not a metaphor. The DNA sequences are alignable, and researchers can trace specific stretches back to the shared ancestor.
Surprising Parallels in How Plants and Animals Function
Beyond the basic cellular hardware, some of the similarities between humans and trees are genuinely unexpected. One of the more striking involves immune defense. Both plants and animals use a class of intracellular receptors called NLRs to detect invaders. In animals, these receptors sense bacterial components that have breached the cell. In plants, a structurally similar family of receptors detects pathogen molecules. When triggered, these plant NLRs assemble into a structure called a resistosome, and recent work revealed that this structure is remarkably similar in architecture to the inflammasomes and apoptosomes that animal NLRs form.8PubMed. Plant and Animal Innate Immunity Complexes: Fighting Different Enemies with Similar Weapons9PubMed. A Comparative Overview of the Intracellular Guardians of Plants and Animals: NLRs in Innate Immunity and Beyond
Whether these NLR systems in plants and animals inherited a shared prototype from the common ancestor or independently converged on the same solution is still being worked out. Either way, the structural overlap is real and deep. Both systems use similar protein domains and similar activation mechanisms to achieve the same goal: killing compromised cells before an infection spreads. Trees cannot mount the antibody-driven immune response that humans can, but their innate immune toolkit is built from strikingly similar parts.
Another parallel involves programmed cell death. When your body needs to dismantle a damaged or surplus cell, it activates an orderly self-destruct program rather than letting the cell simply rupture. Plants do something analogous. When a tree’s leaf cells are attacked by a fungus, the plant can trigger a deliberate death of cells surrounding the infection, walling off the pathogen. The molecular pathways are not identical, but both kingdoms inherited the basic concept of controlled cellular suicide from their shared ancestor, and some of the proteins involved are recognizably related.
Circadian Clocks and Light Sensing
Humans and trees both keep track of time using internal biological clocks, and one of the molecular components they share for this purpose is a family of proteins called cryptochromes. These blue-light-sensitive proteins were first identified in a small plant called Arabidopsis, where they regulate growth and flowering.10PubMed. The cryptochromes: blue light photoreceptors in plants and animals They were subsequently found across the full range of eukaryotic life, including in animals, fungi, and even bacteria and archaea.11PubMed Central. Cryptochromes and the Circadian Clock: The Story of a Very Complex Relationship in a Spinning World
In plants, cryptochromes serve as light sensors that help set the circadian clock and guide development decisions like when to flower. In animals, they play a role in synchronizing the circadian clock to the light-dark cycle.12PubMed Central. The cryptochromes The same family of proteins, descended from a common ancestral gene, helps a tree figure out when to open its leaves in the morning and helps your brain figure out when to make you sleepy at night. There is even speculation that cryptochromes may function as magnetoreceptors in migratory birds, adding yet another sensory dimension to this ancient protein family.
Glutamate Receptors Without a Nervous System
Here is one that tends to surprise people: trees have proteins closely related to the glutamate receptors in your brain. In the human nervous system, glutamate is the main excitatory neurotransmitter, essential for learning, memory, and virtually every form of neural signaling. Plants, which obviously have no neurons, nonetheless carry glutamate receptor-like proteins that play roles in a variety of processes, from calcium signaling to wound responses to pollen tube growth.13PubMed. Comparing Plant and Animal Glutamate Receptors: Common Traits but Different Fates?
The existence of these receptors in plants does not mean trees think or feel in any sense resembling what animals do. What it does mean is that the basic protein structure predates the evolution of nervous systems entirely. The common ancestor of plants and animals already had some version of this receptor family, and the two lineages repurposed it in wildly different directions. Animals built neurotransmission around it; plants used it for internal chemical signaling between cells. Same molecular ancestor, completely different career paths.
Where the Similarities End
For all the shared biology, the differences between humans and trees are enormous and fundamental. The most obvious one is energy production. Plants evolved photosynthesis through a second major endosymbiotic event: an ancient cyanobacterium was engulfed by a plant ancestor, eventually becoming the chloroplast. Animals never acquired chloroplasts. This single divergence reshaped everything about how the two lineages interact with their environment, from their need for mobility to the architecture of their tissues.
The structural framework of cells also diverged sharply. Plant cells have rigid walls made primarily of cellulose, which is why trees can stand upright without a skeleton. Animal cells lack cell walls entirely, relying instead on collagen and other extracellular matrix proteins for structural support. The proteins involved in building these external structures are completely different, even though the internal cytoskeleton mentioned earlier is shared.
And while plants and animals both use hormones for long-range signaling within the organism, the specific hormones are largely different. Plants rely on auxin, gibberellin, abscisic acid, and others; animals rely on testosterone, estrogen, insulin, and their kin. There are occasional overlaps in signaling chemistry. Estrogen receptors turn out to be among the most ancient steroid receptors, and they can bind phytoestrogens, plant-produced compounds that mimic estrogen’s shape.14PubMed Central. Chemical communication threatened by endocrine-disrupting chemicals But these cross-kingdom interactions are chemical coincidences built on top of shared ancestry, not evidence that trees and humans use the same hormonal systems.
Roots and Guts
One of the more recent and thought-provoking areas of research involves the microbial communities that live on and in both plants and animals. Your gut is home to trillions of bacteria that help you digest food, produce vitamins, and fight off pathogens. A tree’s root system is home to an analogous community of microbes that help it absorb nutrients, fend off disease, and communicate with neighboring plants. The parallel is more than superficial. Both guts and roots are nutrient-absorbing interfaces that host large, complex, dynamic microbial communities, and the relationships between host and microbe in both systems share functional features.15PubMed Central. Gut and root microbiota commonalities
In both cases, the microbiome helps regulate host gene expression, supplies nutrients the host cannot produce on its own, provides defense against pathogenic microbes, and undergoes predictable changes as the host develops and ages.16PubMed Central. The root rhizosphere as a functional analog to the gut microbiome: Cases for microbial symbiosis and dysbiosis in parallel contexts Both systems maintain a core microbiome that is taxonomically variable from individual to individual but functionally consistent, meaning different species of bacteria may be doing the same metabolic jobs in different hosts. When the balance is disrupted in either system, the consequences are similar: increased vulnerability to disease, impaired nutrient uptake, and cascading health problems.
The gut-root parallel is not evidence of direct relatedness in itself, since the microbial communities colonizing each system are largely different species. But the fact that both animals and plants evolved the same strategy of outsourcing critical functions to microbial partners suggests that the pressures driving these symbioses may be as old as multicellular life.
Why People Struggle With This Idea
Despite being well-established science, the idea that humans are related to trees trips a lot of people up. Research into biology misconceptions suggests this is not because the concept is especially hard to grasp, but because it clashes with deeply held intuitions about how the living world is organized. People naturally sort organisms into rigid categories, and the boundary between “animal” and “plant” feels so absolute that common ancestry seems implausible.17PubMed Central. Common origins of diverse misconceptions: cognitive principles and the development of biology thinking We see ourselves as fundamentally different from a fern or a fruit tree, and in many respects we are. But “different” and “unrelated” are not the same thing.
Another common stumbling block is the assumption that shared ancestry means recent shared ancestry. When someone hears that humans are related to trees, they sometimes picture a half-human, half-tree creature somewhere in the past, which understandably seems ridiculous. The reality is that the common ancestor looked nothing like either a human or a tree. It was a single-celled or perhaps early multicellular organism, living in an ancient ocean, over a billion years before anything resembling a tree or an animal existed. Humans and trees are related in the same way that two extremely distant cousins are related: there is a real, traceable genealogical connection, but you have to go very far back to find it.
Gene Transfer Across Kingdoms
Beyond vertical inheritance from the shared ancestor, there is the question of whether genes have moved directly between plants and animals at any point since the split. Horizontal gene transfer, where genes jump between unrelated organisms rather than being passed from parent to offspring, is well documented in bacteria but much rarer in complex organisms. Claims of interkingdom horizontal gene transfer to plants have been made over the years, but a recent large-scale reanalysis casts doubt on many of them. Researchers compiled over 1,100 candidate genes that had been proposed as transfers to plants from other kingdoms. When they reanalyzed these candidates with updated methods, only about 29 percent still showed a signal consistent with horizontal transfer. The rest could be explained more simply by gene loss in related lineages or gaps in sampling.18PubMed Central. Reassessing Interkingdom Horizontal Gene Transfer Suggests Limited Influence on Plant Genomes
This matters for the “are we related” question because it helps clarify how the shared genes got there. The vast majority of the genetic overlap between humans and trees comes from vertical descent, passed down generation after generation from the common ancestor, not from any recent gene-swapping event. The similarities are genuinely ancestral. They reflect a shared starting point, not ongoing exchange.
Pigments and Porphyrins
One of the more visually poetic connections between humans and trees involves the pigments that define each organism’s basic biochemistry. Hemoglobin, the protein that makes your blood red and carries oxygen to your tissues, contains a heme group built around a porphyrin ring. Chlorophyll, the pigment that makes tree leaves green and captures light energy for photosynthesis, is built around a nearly identical porphyrin ring. The two molecules are biosynthetic relatives: the early steps in their production are the same, and the pathways diverge only later. Your red blood and a leaf’s green pigment are, at the molecular level, cousins.
Both molecules are built on a tetrapyrrole scaffold, a ring of four nitrogen-containing sub-units surrounding a central metal atom. In hemoglobin, that metal is iron. In chlorophyll, it is magnesium. The shared biosynthetic pathway leading to these pigments is yet another inheritance from the ancient common ancestor, repurposed by animals for oxygen transport and by plants for harvesting sunlight. It is not the kind of thing you would ever guess by looking at a leaf and a drop of blood, but at the chemical level, the family resemblance is unmistakable.