Remember Lord Shiva is the greatest of the Gods. The hierarchy of Shiva, Vishnu, Brahma correlates to cosmic system of Universe, galaxies and solar systems. While Shiva represents Universe, Shivalinga represents process of creation of universe like  Big-Bang model). Similarly Lord Vishnu represents the Galaxies and Swastik represents  the process of galactic and starry creations. On same note, Lord Brahma represents the Solar system, while all terrestrial gods bow Him. This logic is supplemented by the symbols and mythologies about Bhagwan Brahma.

Bhagwan Brahma (not Brahm, the universe) is identified with the Vedic God Prajapati, who controls the terrestrial Gods, such as God of Water, Earth, Mountains, Rivers, Air, Clouds and Thunder - they all were geological phenomenon related to creations of Earth and were personified by different names. These Earthly forces are also treated as Gods in Greek and other cultures. Thus, Brahma being the great grandfather of terrestrial Gods actually represents Solar systems and reports to Bhagwan Vishnu.

Bhagwan Brahma emerged from Vishnu’s naval as said in scriptures. Science concludes that our solar system was born of cloud burst and its gravitational collapse, a constant process in any galaxy. As Vishnu represents Galaxy, Lord Brahma’s birth from Vishnu’s naval connotes that Solar system was born out of large Galaxies. Moreover, he is God of Gods representing other physical entities in the solar system and Earth. Thus, Brahma controls Gods of Sun, Moon, fire, water, winds etc. The formation of fire and water in space happened before the planets. Refer to link(http://www.sci-news.com/space/science-half-water-Earth-older-than-Sun-02173.html). Therefore, figuratively it is said that Brahma created life and send them to Earth.

Bhagwan Brahma’s four head face four directions, representing the spread of planets around the Sun in relatively planer orbits. Four heads either symbolically represent all nine planets or possibly represent four major planets mainly responsible for the stability of solar system. Mythologies say that originally Brahma had five heads. One was chopped off by Bhagwan Shiva to stabilize the Earth. - Siva Purana, Gyanasamhita 49:65-80.

A latest scientific finding does corroborate the above scriptural mention and states that the tenth planet a big one, was ejected to keep the Earth and other planets stable and safe. (Refer to: http://www.universetoday.com/90857/was-a-fifth-giant-planet-expelled-from-our-solar-system/ “Nesvorny and his team performed thousands of computer simulations that attempted to model the early solar system in an effort to test the “jumping-Jupiter” theory. Dr. David Nesvorny found that Jupiter did in fact jump due to gravitational interactions from Uranus or Neptune, but when Jupiter jumped, either Uranus or Neptune was expelled from the solar system. “Something was clearly wrong,” he said.
Based on his early results, Nesvorny removed a fifth giant planet, like Uranus or Neptune from his simulations. Once he ran the reconfigured simulations, everything fell into place. The simulation showed the fifth planet ejected from the solar system by Jupiter, with four giant planets remaining and the inner, terrestrial planets untouched.
Nesvorny concluded with, “The possibility that the solar system had more than four giant planets initially and ejected some, appears to be conceivable in view of the recent discovery of a large number of free-floating planets in interstellar space, indicating that the planet ejection process could be a common occurrence.”). Also Refer to: https://www.sciencedaily.com/releases/2011/11/111110142102.htm

When Brahma was creating the world through creative speech, Goddess Sarasvati was born in his mouth or tongue. Another legend says Dewi Sarasvati sprang from the forehead of Brahma. As Dewi Sarasvati is the Goddess of knowledge, the fable actually represents that the knowledge was created by using brain. As she was born of Brahma, she was symbolically said to be daughter of Brahma, indicatively, not biologically.

Brahma meditated with knowledge and created Kaam Dev, the God of Attractions and mating. Due to this, Saraswati was identified as his wife too, Note that this was also symbolic and not a biological relationship. A parable in Matsya-Purana states that his body was divided into two halves: one male and the other female. Enraptured by his female half (who is Sarasvati) Brahma desired her and together they created the demigod Manu, who in turn created the world. These parables encode certain knowledge which gets reverence and attention by linking to Gods. These are iconological personifications and not biological events..

Note that unlike Bhagwan Rama and Kritsna, no other Gods, such as Bhagwan Shiva, Vishnu and Brahma or Devis were born of father and mother. Therefore, they represent the personification of events and knowledge.