An interesting exercise is to read through a brief introduction to the origin of multicellular organisms, such as the Wikipedia article linked here.
Although a more rigorous analysis of the issues of the origin of multicellular organisms would be found elsewhere, Wikipedia, with its naturalistic predilection, still makes it plan that a scientific explanation is lacking.
When we consider the system-level functionality of even the simplest animals, we can use our imaginations to propose scenarios that might lead to their origin. The Wikipedia article mentions several imaginative proposals:
“Multicellular organisms arise in various ways, for example by cell division or by aggregation of many single cells.”
“One hypothesis for the origin of multicellularity is that a group of function-specific cells aggregated into a slug-like mass called a grex, which moved as a multicellular unit.”
“A unicellular organism divided, the daughter cells failed to separate, resulting in a conglomeration of identical cells in one organism, which could later develop specialized tissues.”
The symbiotic “theory suggests that the first multicellular organisms occurred from symbiosis (cooperation) of different species of single-cell organisms, each with different roles.”
“The colonial theory of Haeckel, 1874, proposes that the symbiosis of many organisms of the same species (unlike the symbiotic theory, which suggests the symbiosis of different species) led to a multicellular organism.”
The oxygen availability hypothesis “suggests that the oxygen available in the atmosphere of early Earth could have been the limiting factor for the emergence of multicellular life.”
“The snowball Earth hypothesis in regards to multicellularity proposes that the Cryogenian period in Earth history could have been the catalyst for the evolution of complex multicellular life.”
All of these imagined scenarios, and others not mentioned, fail to fill in the void with any mechanism consistent with known laws of physics explaining how unguided natural processes resulted in functional biological systems that had never been seen (or imagined) before on Earth.
Imagine a world in which the existence of anything other than single-cell organisms is absent from reality. What natural process, consistent with the action of the laws of physics, would cause single cells to move towards the unimagined goal of differentiating themselves into all of the needed types of cells that then organize themselves into an creature that possesses a digestive system, or a circulatory system, or a nervous system, or an immune system, or a reproductive system?
Does the committed evolutionist unconsciously impute their imagination into the supposed biological outworkings of the laws of nature? Should scientists imagine that a higher partial pressure of a certain gas can cause the origin of complex functional biological systems?